Why won't my winged steed of the Ebon Blade work?

So I made a DK and i'm currently lvl 79 in northrend trying to progress to icecrown but I realized I need flying to really do the content.
So I grinded for ages to buy lvl225 riding and saved the gold for the mount, but even in outland and northrend I can't cast it and use it at all?
It doesn't light up like my other abilities and I'm confused as to why I can't use it?
Any help would be greatly appreciated, thank you.

P.S. It just says "You can't use that here" even though I'm literally in Storm peaks???

did you buy northend flying? It's a license required to fly in northend, bought in dalaran/borean tunrdra (1kgold)
Buy other mount to check if you can fly at all
as a temporary solution, get a free k3 mount (for use in st.peaks, icecrown, schola)

wow thanks heaps man, really appreciate the help!