Getting a List of Sessions
At the simplest level you can either call the get-ASSession cmdlet
PS > add-pssnapin powerSSAS # this only needs to be run once per session
PS > get-ASSession localhost
This can be run from any location (provided you have powerSSAS Loaded
Or if you have created a drive then you can navigate to the sessions collection
PS > add-pssnapin powerSSAS
PS > new-psdrive ssas powerSSAS localhost
PS > ssas:
PS > cd sessions
PS > dir