I have a whole section on charming on my bard guide, as well as how to tab target
http://wiki.project1999.com/Thrasos%...e#Introduction
The charm lasts for 18 seconds, not one pulse (a pulse is 3 seconds, ie the casting time of a bard song).
What you do is select a target, begin the cast. Once you have begin to cast the song you can switch targets. Use your key that is bound to 'cycle through targets' to select the target you want the pet to attack. Once the charm spell lands and the pet window pops up, hit pet attack.
You kind of develop a 6th sense for when you should start recasting your charm spell. If you time it right you can reland a charm spell right after it wears off in order to keep the mobs at a safe distance from you.