There is an faq766-6779 on this topic. You could alternatively:
1) upgrade to CR2008 which allows multiple values
2) ask the user to enter the values at the prompt like this:
'ABC','DEF','GHI'
Then in the command set up the parameter on the right as a string, and instead of enclosing the parm in single quotes, set up the selection clause like this:
table.field in ({?Parm})
-LB