Alternatively you can still compile your dll with .NET 4.0, but create a file called "3dsmax.exe.config" in your Max root dir and paste the following in there:

<?xml version="1.0"?>
<configuration>
  <system.diagnostics>
    <assert assertuienabled="false"/>
    <sources>
      <!-- This removes Binding error messages coming from AdWindows components -->
      <source name="System.Windows.Data" switchName="SourceSwitch">
        <listeners>
          <remove name="Default" />
        </listeners>
      </source>
    </sources>
    <switches>
      <add name="SourceSwitch" value="All" />
    </switches>
    <trace autoflush="true" indentsize="4"></trace>
  </system.diagnostics>
  <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v4.0.30319"/>
    <supportedRuntime version="v2.0.50727"/>
  </startup>
</configuration>

This should force Max to load with .NET CLR 4.0 (instead of default 2.0/3.5 CLR)

Marsel Khadiyev (Software Developer, EPHERE Inc.)