If you're using custom user profiles in Salesforce, you'll need to make sure to give your users access to the objects that are in the WebMerge managed package as well as Visualforce pages.
To update the permissions, go to Setup > Users > Profiles and then click the link under Profile next to the profile that you would like to update.
Scroll down to the Custom Object Permissions section in the settings and you'll need to give the profile Read, Create, Edit, and View All access for the following objects:
- Field Mappings
- WebMerge Documents
- WebMerge Mappings
- WebMerge Mapping Criteria
- WebMerge Mapping Filter
Here's a screenshot of what your settings should look like:
If you've created any new custom visualforce pages, or you didn't grant access to All Users when you installed WebMerge, you'll need to give access to those VF pages under Setup >Visualforce Pages > Security.