To get it working:

1) First install Enderal manually with the old launcher on Nexus in the original Skyrim folder.

2) Then open the Skyrim.ini (in my games folder) and add:

[Archive]
bUseArchives=1
SResourceArchiveList=Skyrim - Textures.bsa, Skyrim - Meshes.bsa, Skyrim - Animations.bsa, Skyrim - Voices.bsa, Skyrim - Interface.bsa, Skyrim - Misc.bsa, Skyrim - Sounds.bsa, Skyrim - VoicesExtra.bsa,
SResourceArchiveList2=Skyrim - Shaders.bsa, Update.bsa, Enderal - Meshes.bsa, Enderal - Misc.bsa, Enderal - Sounds.bsa, Enderal - Textures.bsa, Enderal - Textures2.bsa, Enderal - Textures3.bsa, Enderal - Textures4.bsa, Enderal - Voices.bsa

3) To disable the error warnings: Open SkyrimEditor.ini (in Skyrim folder) and add:

[MESSAGES]
bAllowYesToAll=1
bBlockMessageBoxes=1

German: https://psteamcommunity.yuanyoumao.com/groups/SureAI/discussions/0/358417008728995325/