Access methods declared in interfaces of another project

Question

I have project "X" in C# using .Net framework 2.0 ,I am Adding the assembly created to GAC using utility gacutil and registering it using regasm while doing this I exporting it into a tlb so that i can expose it into a C++ , that works fine.

Now I have added reference of this assembly(X.dll) in another C# project "Y" which uses .Net framework 3.5.Here I can see the namespace and interfaces and classes but whiel trying to access methods declared within interfaces ,no method seems to be accessible in it.

What I am missing?