Can't put rune on my armor

I am a w/e and i am unable to apply a minor air elemental rune on my glove piece of my armor.
why? i double clicked the rune and click on the glove, but nothing happened.

you can only use runes that apply to your primary profession

you have warrior armor not elemental armor

This should be stated much more clearly to new players. I had no idea. This makes hitting that 13/14 expertise breakpoint on a ?/R impossible.

Not to mention that */Rs don't get Expertise.

Secondary Rangers do not get Expertise.

It should be made more clear to new players though....I just bought a minor death for 350...but I'm an Elem...oh well.