register a DLL to work with 64 bit OS

Hi, I have 64 bit windows 7. I created classlibrary (VS2012) and SN/registered it with

C:\>sn -k C:\API3\test\bin\Debug\mykey.snk
C:\Windows\Microsoft.NET\Framework\v4.0.30319>regasm C:\API2\test\obj\Debug/test.dll /tlb /codebase

it registers  just fine and ASP can create the object just fine (IF I set APP POOLING to 32 bit=true.)

But how can I register this dll for 64bit version? 

I have tried to change the "target CPU:" under compile to x64 but when I want to register the DLL I get " in not a valid .DLL". I even tried regsvr32 but no luck. any suggestion?