Hi:
Look at the example of prompting the user for a REGION value in order to run a SP for a specific region...in this paper:
http://www2.sas.com/proceedings/forum2008/024-2008.pdf
The macro variable ® is used to hold the selection that the user made from the stored process prompting interface. Even though there is only one parameter in the stored process example in this paper, a definition for a second parameter is easy to do. This paper shows the use of ® in a WHERE statement for row selection purposes. However, you could just as easily have used a selection in a VAR statement or a COLUMN statement or a TABLE statement to control the columns that appear on a report.
The best recommendation I know of is to start with a working program (one that is NOT a stored process) that does what you want to do and then gradually use SAS macro variables to take the place of pieces of code (where you will subsitute the user's choices for code portions). Then once you have the revised code working with your "hard-coded" macro variables, you are ready to turn this program into a stored process and to register the stored process and parameters in the metadata.
When a user is prompted to supply information to a stored process, the prompting interface creates global macro variables which are passed to either the Stored Process Server or the Workspace Server for use when the SP is executed on the server.
To understand more about SAS macro processing and macro variables, this paper provides a good introduction:
http://www2.sas.com/proceedings/sugi28/056-28.pdf
cynthia