The Partner Source download functionality is currently in transition and you cannot download from the Partner Source site. The good news is there is a work around. Often you need to get a cumulative update for NAV, so here is how to get them during the transition.
Sign into Partner Source (mbs.microsoft.com/partnersource)
Find the update you are looking
Note the ID for the cumulative update
Go to support.microsoft.com
Search using the ID from above.
Select Hotfix Download Available