desertfusion

Best answer as chosen by Asker / Ratings
To wear mithril armour, you must raise your defence level to 20, to wear mithril weapons it's 20 attack. If you have 10-19 defence, I recommend buying a black armour set.

Rangerman

In order to wear mith armor, you need 20 defence. For advanced info, you need 30 for addy and 40 or 50 for rune.

godzilla2y

The way I read this question, I took it that you bought the armor set but now can't use it. If this is the case, you need to go to the grand exchange, right click on one of the grand exchange clerk, and click sets. After this has been done, scroll through the different armors that are in sets, and find mithril armor. Be sure to have the armor set that you bought in your inventory, then click trade or whatever it is... I sadly can't remember right now; but if what you mean by that you can't use it is that you don't have the required levels, then my fellow supercheats members have hit that spot on.

hutinghan

There might be 2 reasons. Either you don't have defense level 20 (you need to level up), or you didn't exchange your set for the items. To do that you need to go to Grand Exchange and right-click on a Grand Exchange clerk and select Sets. Then right-click your set and select Exchange. Hope this helped.