Don't forget to reference the required namespaces:

using Autodesk.Max;
using Autodesk.Max.Plugins;

First lets define our action item:

internal class MyAction : ActionItem

{

       const string ActionName = "My Action";

       public override string ButtonText

       {

              get { return ActionName; }

       }

 

       public override string CategoryText

       {

              get { return Plugin.ProjectName; }

       }

 

       public override string DescriptionText

       {

              get { return ActionName + " " + Plugin.ProjectName; }

       }

 

       public override bool ExecuteAction()

       {

              // execute me

              return true;

       }

 

       public override IMaxIcon Icon

       {

              get { return null; }

       }

 

       public override int Id

       {

              get { return 1; }

       }

 

       public override bool IsChecked

       {

              get { return false; }

       }

 

       public override bool IsEnabled

       {

              get { return true; }

       }

 

       public override bool IsItemVisible

       {

              get { return true; }

       }

 

       public override string MenuText

       {

              get { return "&" + ActionName + "..."; }

       }

}

Then we declare some class variables that we will keep track of. These are stored inside out IPlugin-derived class.

IIMenu menu = null;
IIMenuItem menuItem = null;
IIMenuItem menuItemMyAction = null;
uint idActionTable = 0;
IActionTable actionTable = null;
IActionCallback actionCallback = null;

We need to declare one additional 'dummy' class to enable our action to execute:

public class ZKActionCallback : ActionCallback
    {
        public override bool ExecuteAction( int id )
        {
            // Do nothing...
            return true;
        }
    }

Inside the IPlugin.Initialize() method we call MenusAndActionsRegister(), which is defined as follows:

void MenusAndActionsRegister()
        {
            IIActionManager actionManager = global.COREInterface.ActionManager;
            IIMenuManager menuManager = global.COREInterface.MenuManager;

            IActionTable actionTable = null;
            // Set up global actions
            {
                idActionTable = (uint)actionManager.NumActionTables;

                string actionTableName = ProjectName + " Actions";
                actionTable = Global.ActionTable.Create( idActionTable, 0, ref actionTableName );
                actionTable.AppendOperation( new ActionItems.MyAction() );
                actionCallback = new MyActionCallback();

                actionManager.RegisterActionTable( actionTable );
                actionManager.ActivateActionTable( actionCallback as ActionCallback, idActionTable );
            }
            // Set up menu
            {
                this.menu = menuManager.FindMenu( ProjectName );

                if( this.menu != null )
                {
                    menuManager.UnRegisterMenu( menu );
                    global.ReleaseIMenu( menu );
                    this.menu = null;
                }

                this.menuItemZookeeper = global.IMenuItem;
                this.menuItemZookeeper.Title = ProjectName;

                // Main menu
                {
                    this.menu = global.IMenu;
                    this.menu.Title = ProjectName;
                    menuManager.RegisterMenu( menu, 0 );

                    // Launch option
                    {
                        this.menuItemMyAction = global.IMenuItem;
                        this.menuItemMyAction.Title = "&MyAction";
                        this.menuItemMyAction.ActionItem = actionTable[0];

                        menu.AddItem( this.menuItemMyAction, -1 );
                    }
                }
                this.menuItem.SubMenu = menu;

                menuManager.MainMenuBar.AddItem( this.menuItem, -1 );

                global.COREInterface.MenuManager.UpdateMenuBar();
            }
        }

Finally, when unloading our plugin it is preferrable that we also unregister all the menus. We do this in the IPlugin.Cleanup() method by calling the following function:

void MenusAndActionsUnregister()
        {
            if( this.actionTable != null )
            {
                this.Global.COREInterface.ActionManager.DeactivateActionTable( actionCallback, idActionTable );
            }

            // Clean up menu
            if( this.menu != null )
            {
                this.Global.COREInterface.MenuManager.UnRegisterMenu( menu );
                this.Global.ReleaseIMenu( menu );
                this.Global.ReleaseIMenuItem( menuItemMyAction );
                this.Global.ReleaseIMenuItem( menuItem );

                this.menu = null;
                this.menuItem = null;
            }
        }

And we're done!