To add solutions you need to be local administrator on top of shell
admin. Run the shell as administrator.
As I said in comments, i cant find a specific requirement specified on
TechNet, but I did a test on one of my farms:
•First I added a brand new user to my AD.
•As being member of the AD the users only right on my farm server is
that its part of domain users, and hence part of local User group.
•I fired up a SP2010 administration shell using Run as different user
(right click on shell) and logged in as my new user. Already as I
started up i got some errors because of lack of access (The local farm
is not accessible. Cmdlets with FeatureDependencyId are not
•I tried adding solution and got:
Add-SPSolution : Cannot access the local farm. Verify that the local
farm is pr operly configured, currently available, and that you have
the appropriate permi ssions to access the database before trying
•I now gave test user farm admin privileges and started up a new
shell. Still got the same error.
•I ran another Shell as my farm admin (if logged in as account
remember to use Run As Administrator if you have UAC enabled). Here I
added my test user as Shell Admin
•I now started a new shell with my test user. No errors while
loading, but when i tried adding I got the same error as you:
Add-SPSolution: Access Denied
•I then added the test user to the local administrator group and
opened up a new shell.
•When adding the user I could add the user without issues.
Source: Accepted answer to the question Sharepoint 2010 powershell add-spsolution access denied