SOLUTION FOUND

I found the answer here on the MS SQL Server Forum for Analysis Services:

Solution for: OLE DB or ODBC error:Login failed for user NT Service\MSSQLServerOLAPService error - create new login in the SQL Server database engine so analysis services can read the data in the database being analyzed.

Create login and grant access:

Open up SQL Server Management Studio [login to the database engine]>

Security >

Login (Right? click) >

New Login >add user: "NT Service\MSSQLServerOLAPService">

select the database being analyzed in the drop-down list at the bottom >

User Mapping [click on the database in the list] >

In the window database role membership click on db_datareader [this grants access to read the data]

More details here http://msdn.microsoft.com/en-us/library/ms189751.aspx

Hope this helps!

Shahfaisal Muhammed http://shahfaisalmuhammed.blogspot.com

http://shahfaisalmuhammed.blogspot.com/2012/09/login-failed-for-mssqlserverolapservice.html

Thank you everyone who helped me - thank you very much!