depends on how you getting the parameters. I am assumming that you are entering the multivalued parameter from the Crystal parameter input page. If so there are a couple of ways to deal with it.
1) don't pass the parameter the stored proc, let it return too many records (this requires modifying the stored proc) and then filter the results in the Record Selection Formula section by checking if the returned value is in the array of selected values.
2) pass in a delimited string to the stored proc and let it parse the values and filter that way. So for a list the user might enter a comma delimited string.
Again, I am assuming that by multivalue you mean a user can select something like mulitple divisions in a company.
Hope this is on the right track.