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.
To update the permissions, go to Setup > Manage Users > Profiles and then select the profile that you would like to update. Click the Edit button.
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: