Strong named assemblies cannot reference assemblies which aren't strong named.

Decompile with ildasm and recompile with ilasm using your key.

Default ILDASM and ILASM locations as of .Net 4.0

"C:\Program Files\Microsoft SDKs\Windows\v7.0A\bin\ildasm.exe" ClassLibrary.dll /out:ClassLibrary.il
"C:\Windows\Microsoft.NET\Framework\v4.0.30319\ilasm.exe" ClassLibrary.il /res:ClassLibrary.res /dll /key:myKey.snk /out:ClassLibrary.dll