I had the same trouble. Finally have it loading, but not sure why it is working. I can list what I did that has gotten it to load. More detail then needed, but after many attempts I have not isolated the “fix” in this list.
copied the entire DeadLine2 directory into \addons
used the deadline mission file, but edited it with my map “Try” and only kept survival. Left the rest as is.
copied \materials\vgui from DeadLine2 into \addons ry
modified the addoninfo as shown below
Lastly, when I load l4d2 I first check the Extras>Addons and click on “Try”
Then load >survival >local server and choose “Try” under Addons
No troubles after that. Of course, maybe it was my poster file or map icon, so now it just shows the deadline images, but at least my map loads for testing.
addontagline “Catch the last subway… out of hell… Again!”
// addonSteamGroupName “tryadd-on”
// addonauthorSteamID “TTz1”