This thread was a long time ago but for sake of response I'll give you one if you haven't found out how to fix it yet.
Problem 1 Fix: You must go to the Triggers, add a new one with Ctrl+T, then make your event (yellow flag at top) to Map Initialisation. Then add a new Action (green cinema card next to the flag) and search for Set Alliance. Then set the alliance between the three human players: Make Player X and Player Y treat each other as Ally with Shared Vision and Pushable.
Problem 2 Fix: You must follow these steps:
1. open Left 2 Die map from Blizzard, ignore error. Put a Stank, Spotter, Hunterling, Choker, Kaboomer on the map. They are white domes right? Go to step 2.
2. Go to dependencies and then double click on Left2Die.SC2Mod
3. A window will pop up, go to the search bar and look for the Left 2 Die mod.
4. Click on it, make sure the Data Download says 100%, the green bar must be full.
5. Press OK, OK, when you go back to the Dependency popup Left 2 Die mod will not have a file path. This is ok.
6. Check on the special units you added, they should all have their unit models which means everything is working correctly. If not then delete them and place them again, the mod data should load then.
Problem 3 Fix: You must make a new Mod folder under where your game is installed. For me it is C:\Program Files (x86)\StarCraft II and I have a Mods folder inside StarCraft II.