This is similar to already created thread here: Mixed mode assembly in .NET 4
Using the app config, I was able to force the assemblies to run on .NET 4. On an XP Mac
You're experiencing that problem because .NET 3.5 uses the Common Language Runtime (CLR) version2 and .NET 4.0 runs on CLR v4. Therefore if your assembly was built in .NET 3.5, it will only run on a computer that has CLR v2.
Long story short. Compile your .NET 3.5 assembly as a .NET 4 assembly, otherwise install .NET 3.5 on the target computer as well.
You can see this site for more information: