Now i know there are some different guides how to downgrade/unpatch from 2.4 to 2.3. But i tried some of them and it didn't work for me, so this is a guide how to do it differently. (THIS DOES NOT WORK ON MAC, sorry).
1. Copy the file world of warcraft in C:/program files and copy it to 'program files'(the same file). This is not neccesary but i usually keep an backup. And call it what ever you want, like 'world of copy' or 'world of shit' whatever you want, but something you can easily remember.
2.Now when you have an copy of the file go in to it and delete the 2.4 patch (I think it's called '2.3.3 to 2.4') both the tornado thing (blizzard downloader) and the purple shooting thing (enGB-patch). And then delete WoW (not the Launcher)
3.Look for the repair tool inside the copied file and repair(the button at the top corner). This may take a while.The repair tool will automaticly downpatch your WoW to 2.0.
4.Now look for the blizzard dowloader (tornado thing) for patch '2.3.0 to 2.3.3'. Open it and it will download the Zul'Aman patch and once it is done your WoW will now be 2.3.3.
Now open Wow (Not the Launcher, it will try to download patch 2.4). And if everything is done correctly you will see in the bottom left corner in the WoW login window 'Version 2.3.3'. And you are ready to go on to private servers with 2.3.(Remember to cange the realmlist to whatever server you want to connect to)
Hope this guide works for you, remember, dont start "World of warcraft launcher". If you want to change back to Blizzard servers just go in to your regular map (Not the copy you made) in the program files (C:/program files/world of warcraft).
If this doesn't work, i suggest that you use one of the other guides.
Hope this works for everyone out there trying to get on the private servers!