1.right click the project property.
2.under Debug->start action set "start external program" property to the nunit's path
3.run->cmd->c:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\Bin\) sn.exe -k mytest.snk
4.under Signing->Subscribe Sign the assembly and then select mytest.snk.
it's ok now ,you can run the project and debug the programme.