That's odd, I tried your repose macro and it works just fine for me...

Also, you could do /ac "name of ability" <tt>, that'll cast your spell on target of target. So you could cast virus on the enemy the tank is currently tanking without having to switch off of the tank. There is no point in doing that with sleep however, since the tank will just wake it up :P

If you use a mouse, you can click on the names of enemies in the enemy list. Red dot means you have the most hate from that enemy and you are their current target, so those would be the mobs you want to sleep first. Yellow means you are high on the hate list, and green means you are low on the hate list. You should put your enemy list somewhere close to your action bars or party list, somewhere where you'll see it, it's very useful for white mages ^^

Hope that helps... sorry I couldn't help fix your macro :P