what you need:
Menu Menu Menu App http://forums.halomods.com/viewtopic.php?t=58251
Entity http://forums.halomods.com/viewtopic.php?t=67671
(I use entity 1.3 AI fix with 1.4 plugins)
a) Make a new folder that contains the following:
1. The modded map (I will be using night-OPS by Tbeezie for this tut)
2. The mainmenu that you want to add maps to
3. Shared map (so we can open the map in entity)
4. The DDS preview (optional)

b) Open you modded map in a hex editor/workshop (I use HXD hex editor). Only Overwrite characters
1. Scroll down a little until you see the file name (red) and the scenario name (yellow). Change these to whatever you want. Remember them.
2. save

3. Change the name of the modded map file to what ever you hexed it to

c) Close the hex editor and open up your mainmenu in the menu menu menu app.
1. Select multiplayer
2. Select a blank slot
3. Inject you DDS image (optional)
4. Give the map a name
5. Give the map a description
6. type the new scenario name that you hexed in step b)2.
7. click generate values and take note of what the values are
8. click save all in the top right hand corner

you should get something like this:

d) close the mainmenu app and open entity
1. go to scnr (scenario) and select its only tag.
2. click on the tools tab and click meta editor
3. scroll all the way down to the bottom and find the "Map ID" and the "Next Map ID" values. They will be under map descriptions/multiplayer.
4. Change these values to what ever was generated in the mainmenu app.

5. save it
Hope this helps and happy halloween