Hi,
Are you uploading a new document or linking an existing one? If you're uploading the document from the opportunity screen, it should be linked to it automatically. On the stock screen there is no ability to link the opportunity if you're uploadind a new document. If you're linking a n existing document, the opportunity should show in the dropdown for Opportunity Name. Below are two screen shots, one for uploading a new document and one for linking an existing document:
New Document:
Link to existing:
You should see either of these screens when attaching a document to an opportunity. Are you seeing something different? Was it customized?
- Adam