1. How to install custom maps in OpenCoop or LMS4?

Just drop the map file (ex: occm_inhell.pk4) in your mod folder (ex: for opencoop, ...Doom 3/opencoop/), the mod should detect the map automatically

For OpenCoop, if you wanna host a listen server, you must select the oc_customs map type when you're going to host

2. I can't host Doom 3: ROE campaign (OpenCoop or LMS4)

In order to host Doom 3: ROE maps (campaign and also custom maps that require ROE) you must edit your launcher and add: "+set fs_game_base d3xp"

+ Non-steam users:
Create a new Doom 3 link from the original Doom3.exe and do the next.
For Opencoop: edit the launcher options and add at the end +set fs_game_base d3xp +set fs_game opencoop
For LMS4: edit the launcher options and add at the end +set fs_game_base d3xp +set fs_game lms4

+ Steam users:
Right click in your Doom 3 game, select properties and click in Set Launch Options.
For Opencoop: add +set fs_game_base d3xp +set fs_game opencoop
For LMS4: add +set fs_game_base d3xp +set fs_game lms4

3. How to play ROE custom maps in OpenCoop or LMS4

Read "2. I can't host Doom 3: ROE campaign (OpenCoop or LMS4)"