Try this:
Code:
/cast [@focus,exists,nodead][] Throwdown
This will work if you have a focus which is in range, or if you don't have a focus.
If your focus is out of range, you're probably still out of luck, that's one of the decisions a macro can't make for you.
Better yet may be
Code:
/cast [@focus,exists,mod:ctrl][] Throwdown
This will do the throwdown on your focus (if you have one) only if you've got the control key hit too. If you're clicking, change "mod:ctrl" to "btn:2" and right-click your macro to get your focus throwndown.
Note that this version still has the "focus exists but isn't in range" issue - you can't work around that automatically.