Hey..
I actually had to do this yesterday.
If you have an action button as a URL link - code to
Action - go to URL
Heres an example of mine
="http://ohtwi1sql002/ReportServer_BGCO/Pages/ReportViewer.aspx?%2fDEV+AND+QA%2fDirector+Scorecard%2fBGCO+Summary+pdf_print&rs:Format=PDF&year=" + Parameters!year.Value
Note parameter name starts at &year.
HTH