Can't figure out how to use Mecha Legs

I've been excited to get to Level 21 because I thought with the specialization unlock I could get Mecha Legs for the 25% run speed bonus (this is my first character).When I leveled I pretty much just had enough Hero Points to train all the way around to Mecha Legs but I can't figure out how to use it.

The rifle skills use up all of my left slots and in my right slots I have Elixir H, Call Worm, Call Owl and Rifle Turret. When I click any of the little up arrows on the skills to the right to try to replace them the only things that are available are just the 3 or 4 skills I'm already using. No Mecha Legs.

Anyone know what I might be doing wrong?

Sorry, I don't even know what that means. I go to the Build Tab, but the only place I see Mecha Legs is in the first little specialization area and I can't click it or drag it anywhere.

Can I just put Mecha Legs as one of the skills on that right bar (like instead of Call Owl) or is that not the same type of skill?

Mecha Legs is a trait. Traits can't be clicked like a skill on or moved - they are always active parts of your build https://wiki.guildwars2.com/wiki/Trait

You need to make sure you have the inventions traitline (also known as a specialisation) aequipped first and select that skill as part of that traitline within the build tab

Oh, I just realized that I have it passively. I thought I had to have it on my bar for it to work. Thanks!