In this release of Report Builder there is no direct support for referencing parameters in the Formula dialog. They will show up if you edit a prompted filter condition as a formula, but there is no way to directly insert a parameter reference.
However, with a little trickery and relying on some behavior that is arguably a bug J, you can get one (or even two!) parameter references into the formula:
1. Add a field to your filter
2. Click the field name and choose Prompt
2.5. Do this if you want two references: Click the operator (usually “equals���) and choose From…To
3. Click the field name and choose Edit as Formula