If you have Overload on Steam or GoG, you should just be able to launch olmod.exe from anywhere. If it does not work for you immediately, uninstalling and reinstalling Overload should fix it.
Unzip the olmod package into the same directory as Overload. This directory is the same directory that contains the Overload executable as well as the subdirectory Overload_Data. You can then run olmod from within the same directory as Overload.
Getting Started Playing Online
To begin playing Overload online from within olmod:
1) Click "Play Multiplayer".
2) Click "Internet Match".
3) Click "Join Match".
4) Enter the IP address of the server, found on the Home
5) Click "Join Match" again.
To start a new multiplayer game from within olmod:
1) Select a server to play on from the Server Browser on the Home
page. Be sure a game is not currently running on it!
2) Click "Play Multiplayer".
3) Click "Internet Match".
4) Click "Create Open Match" for a match where players can join the match at any time, or click "Create Match" for a closed match where no one else can join once the game starts.
5) Adjust the game settings to your liking.
6) Click "Create Match" again.
7) Enter the IP address of the server.
8) Click "Create Match" again.