You can't pass multiple values in a param to a stored proc.
What I have done in the past is put the part of the report that calls the stored proc into a subreport. In the main report I then parse out the array of parameter values and concatenate them into a single comma-delimited string that has a comma on the end. That string gets passed as a link into the subreport and thus into the stored proc.
The SP then uses InStr(param, field||',') > 0 to determine whether the data is in the list of params. I use the commas because there are situations where one of the values in the parameter string may be a "piece" of the data in a field and I want to make sure that I'm comparing to ALL of the data in the field. For example, if the user selects "ABC" and I have records with "ABC", "1ABC", "ABC2", etc., I want the record that has only "ABC" in it. Make sense?
-Dell