>I have a form with a private data session. In the form I allow the user to set a variable as a filter for a report. But I can't do a set filter in the form since it is a private datasession and report has the view already as part of it's datasession. So,my question is, in the report where can I set the filter before the report is run?

Have the form manage all the tables, views, relations and filters. Just make sure your report uses a Default DS instead of private. If you do this, the report will default its DS to that of the form as long as it is the Form that starts the running of the report.