You need to adjust your shifter guide plate, it limits the range of motions of your shifter. If you're having trouble finding the gate to get into 5th/6th the plate is not in the correct position. Joe@ZSpeed has a post with instructions I believe, but however I cannot find it.
Here is a similar process for the 350z:
HOW TO: take of shift knob + adjust shifter guide plate - MY350Z.COM Forums
The instructions can be found in the service manual and are crystal clear. For the 2010 Z, TM-24, step 6. Very DIY-able.