you could chance the name in it every time or put the tank as focus then cast MD on the focus (this is my preferred option)
Code:
#showtooltip Misdirection
/clearfocus [button:2]
/cancelaura [button:2] Misdirection
/stopmacro [button:2]
/focus [help]
/stopmacro [help]
/cast [exists, @focus] Misdirection; [exists, @pet] Misdirection
right click will clear the focus (just in case i need to), click while you have a friendly target will put him on focus, clicking while you have an aggressive target (or no target at all) will cast MD on focus or on your pet if you have no focus
but if you really want to type:
Code:
/targetexact "tank name without quotes"
/cast Misdirection
/targetlasttarget