I just got the Black Mage Dressphere but I can't use it?
What's up with that?

You have to learn at least one of the black magic skills at first. Because as far as I remember, you can't use it, if it's brand new and haven't learn anything yet. ;)

Have you equiped into a Garment Grid. If not than you must equip it into an empty node in a Garmnet Grid. From there you should then be able to equip it.

Ok thanks for the help I'll try that.

this may sound stupid but how do i equip it to the game grid

I believe it's the options in the menu marked "Equip" and "Garment Grid." (not "game grid") Oh, and you know, Shinra on the bridge of the Celsius has tutorials that tell you about all this stuff.

Or you could use the instruction manual.