How do I register a DLL file on Windows 7 64-bit?

Well, you don’t specify if it’s a 32 or 64 bit dll and you don’t include the error message, but I’ll guess that it’s the same issue as described in this KB article: Error Message When You Run Regsvr32.exe on 64-Bit Windows

Quote from that article:

This behavior occurs because the Regsvr32.exe file in the System32
folder is a 64-bit version. When you run Regsvr32 to register a DLL,
you are using the 64-bit version by default.

Solution from that article:

To resolve this issue, run Regsvr32.exe from the %SystemRoot%\Syswow64
folder. For example, type the following commands to register the DLL:
cd \windows\syswow64 regsvr32 c:\filename.dll