Apparently Garchomp should be able to have Rough Skin and know Endure. http://bulbapedia.bulbagarden.net/wiki/Endure_(move)#Generation_V_2 But I'm getting an error saying it can't know a past gen Tutor move and have Rough Skin: Apparently older versions don't have this problem, and there are people able to run the combination on the ladder. I am on 2.6.0 since 2.6.1 isn't out for Mac so... inb4 Mac problem again.
I'm unsure if it's Mac related since Zorodark had the same issue on Mac http://pokemon-online.eu/threads/endure-rough-skin-garchomp.31658/ Have you tried importing team, or starting with Ability then move, or move then abilty? ( maybe it works who knows lol ) I seem to recall it got fixed for him also, so might ask him in server
This isn't really Mac-realted, but a bug in version 2.6.0. The only way to fix this for users with version 2.6.0 is to edit the move-files manually. I will try to make a short instruction how you can edit it on your own (need to finish stuff for school first :x)
There is an updated Mac client for version 2.6.2 (thanks to @Duster for creating it) : https://github.com/jinorae/pokemon-online/releases/tag/2.6.2 It should be fixed in this version