I have the same problem.
Bought Premium, got the Battlepack with the Precision Knife, opened it but cannot equip it and it's still missing as a "locked" weapon both ingame and on Battlelog.
I contacted Live Support but the advisor told me they couldn't do anything and I should post the Question to the Battlelog Forums where I got pointed here. Wow.
There were *several* BF4 updates since I opened the Battlepack 10 months ago but still nothing made it available for me.
It's the final missing weapon (for now) and I'd really like to have this issue fixed :/
ceno