Symptom
A proofing gallery at /proofing/gallery-name/ returns Page not found.
Cause
The rewrite rules were not refreshed after Pro was activated, so WordPress does not know the /proofing/ base yet.
Fix
Open Settings → Permalinks and click Save Changes. Since 3.6.0 the plugin repairs this itself on the next admin page load, so on a current version the page usually works after one visit to the dashboard.
The feature itself is described on Proofing (Pro).