I just tested this and the current install of PowerSSAS works (for me at least) on Windows 7. So I can run the same add-pssnapin command that worked on Powershell v1 and Windows Vista. Does the machine you are installing on have the SQL client tools or at
least the AMO library installed? If it does the following should work on Windows 7
import-module "c:\program files (x86)\powerssas 2008\powerssas.dll"
I'm running 64 bit Windows 7, if you are running 32 bit you won't need the (x86) in the path.
I have also extended this concept to create a shortcut on my desktop with the following, that loads powerssas and creates a drive for me. (the v1.0 is in the path for backward compatability, it is actually running v2)
%SystemRoot%\system32\WindowsPowerShell\v1.0\powershell.exe -NoExit -command import-module 'c:\program files (x86)\powerssas 2008\powerssas.dll'; new-psdrive ssas powerssas localhost\sql08