At this point I would manually download the mod using a browser and store it somewhere that you can easily remember and find (say on the desktop as an example). Run Mod Organizer and click "Install a new mod from an archive" (the CD on top of a box icon at the top), browse to where you downloaded the mod, and follow GDSFan's instructions for installing the mod. After the mod is installed, do this:
Double click the mod to open the mod properties dialog
Click the Filetree tab
Rename NeutralVertibirdsEN.esp to NeutralVertibirds.esp
Click Close to close the dialog. You must do this because the other tabs do not automatically refresh when you rename a file.
Double click the mod again to open the mod properties dialog
You should now see NeutralVertibirds.esp in the Available ESPs list at the bottom
Move NeturalVertibirdsDE.esp to the Optional ESPs list at the top
You might also consider getting Mod Organizer 2.1.4 Dev Build 5 from #builds on discord because the devs have fixed a lot of download issues in this build, but let's get this issue resolved first.