it says "Nexus Mod Manager has been temporarily disabled
Disabled = NOT WORKING.
Temporarily = Not permanent
Click on 'off line' to run it off line - You do not need to log in to work off line.
You will be able to do anything that does not require going online - That means it cannot check to see if there are new versions, you cannot download with manager - You CAN download manually - and install using the NMM function 'install mod from file'
NMM doesn't care whether the mod was installed through the download with manager or the install from file - it will still treat the mod exactly the same