Could you verify that the user has followed the necessary steps? As Journal Manager, under Roles, you should be able to find the reviewer's user; click "Sign In As", and you will assume their identity. Now go to User Home, choose Reviewer, and find the submission in question.
If the reviewer has not yet accepted the review or has declined it, they will not be able to upload a file. Likewise, if the Editor has cancelled the review, the upload field will be disabled.
Open Journal Systems Team