What exactly is giving you trouble? You have the diagram. Remove the plastic cover (pops off easily) and then remove the three exposed bolts with a 12 mm socket. Was this thread really necessary?
As Ghost mentioned, corrosion might make it difficult. But the battery compartment is pretty well protected from the elements. I've got 100K miles on the Z and my second battery is nearly 4 years old at this point and everything is reasonably clean, as seen in the pic.
__________________
2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |