So, you have Mithril, but it's just not selectable in the fuse menu? That's bizarre. Well, you might've just missed it, but otherwise that's definitely a bug. No clue you'd go about fixing that, sorry.
If you just got the Mithril, my best suggestion is to load a save before you got the mithril, if possible, and see if getting it again fixes the issue. If not, you might try getting another mithril, and seeing if having a second one "updates" it somehow and lets you see it?