The ASRock Game Blaster should work in any of the PCI Express slots, it will certainly do no harm to try it. It will either work or it won't. I haven't been able to ascertain why ASRock says you must use only one particular slot when there's six of them.
One of the slots is PCI-E x1, the other five are all PCI-E x16, so maybe the Game Blaster will only work in the PCI-E x1 slot, and there's only one of them so you've no choice. Just guessing on that due to lack of specific information which I can't find one there website. No surprises there. I bought an ASRock mobo last year and the manual they supplied with it was for a totally different model. Good job it's downloadable.
All you can do is try it in another slot and see if it works...