From MEPIS Documentation Wiki
Mepis Community Repository
In addition to the repositories that the Mepis developer lists by default, users also can enable software repositories maintained by volunteers from the Mepis user community. For more information on using the Community Repo, please see this wiki page:Community Repository
Adding and Enabling Repositories via Synaptic
The preferred method of adding a repository is through the Synaptic package manager. In Synaptic Menu Bar --> Settings --> Repositories will give you a new window listing your current repositories. The ones with with a check mark in the Enabled column are active. Do not enable a repo unless you have a reason to install packages you know are contained only in that repo. Some repositories contain unstable or testing packages that may not be dependable unless you know how to tweak or troubleshoot any problems that arise.
Adding a New Repository
To add a new repository that is not listed in the window, start by clicking the New button in the lower left. The greyed-out section of the window at the bottom will become enabled for input of the following fields:
Obtain the correct current info from the Community Packaging Project page on MEPIScommunity.org. Here is an example screenshot showing where to place the fields correctly.