When I do group instances I often have trouble picking out  enemy targets because the front lines are so crowded with my party members or totems.  It's really frustrating when you're standing there not doing anything because you can't find a target.   

Is there a way to write a macro to select spell targets by "mousing" over them without having to "click" on the individual target?  Can someone offer anything that would make this simpler?

Unless you changed it tab is the default 'cycle nearest enemy' button.  If your group leader marked a skull then tab until you get to it.  If he didn't then tab to the one taking the most damage.

Best of luck!

TAB auto select closes bad guy

I think what you're getting at is can you cast a spell at someone, by mousing over them? the answer is yes. say you're a healer, but wanted to have the boss targeted. you would use this:

/cast [target=mouseover] <Insert healing spell here>

you would then bind that to a key. put your mouse over the tank, hit the key, and BAM you just healed the tank without anything targeted.