We need to narrow down the actual issue first.
You should be able to see the locking mechanism that holds the seat itself.
With the seat off, snap down the mechanism as if the seat was on. Now try your key and see if it opens easily.
If NOT, you know the mechanism / cable / lock cylinder is gummed up... You can separate the cable from the lock cylinder to narrow it down.
Then, depending on where the issue is, brake cleaner, then maybe some reg oil on the mechanism/inside the cable.
IF the mechanism open's easily as designed, as posted above, you can push down on the seat to get some stress off the mechanism.
OR, you can SHIM the hook just a tad so it's not so tight.
As a side note, make sure the FRONT plastic TAB on the seat is indeed inside the frame holder (next to the fuel tank). It's not difficult to miss that front "hole".
I put just a tad of grease at that spot so the seat goes in easier... And make sure all the rubber bumpers are in place (attached to the seat).