First, think through your expression. If a person with [Full Access] logs in,
Get ( AccountPrivilegeSetName) <> "[Full Access]" will be false, but:
Get ( AccountPrivilegeSetName) <> "Admin Staff" will be true. Thus this simplifies to True Or False which in turn evaluates to true and the find is performed. If you work this through with "Admin Staff" then you also get a true result as the first part of the expression will be true.
Change OR to AND in your rexpression and then your script will perform the way you want it to.
As you have discovered, you can't enter a calculation expression as criteria for a manual find and have it evaluate. This can be done with scripted finds either with the variable as you mention or by using set field steps. I prefer set field as this lists the criteria in the script in a way that doesn't require double clicking the Perform Find step just to see the criteria used.
Enter Find Mode //clear the pause check box
Set Field [Table1::Group ; Get ( AccountPreivilegeSetname ) ]
Set Error capture [on]
Perform Find