If it's exactly as you typed, you need a space between the command and the macro condition, otherwise WoW will try to find a command that doesn't exist.
To clarify the macro, I'd use something like this
Code:
/assist [@focus,help][@pet]
/cast Hunter's Mark
/startattack
/petattack
Telling it to attack the new target is sufficient enough if you don't have it in assist mode. The only reason you would ever need to tell it to stop attack on the old target is if you wanted to clear its threat list. This is only useful to call it off from attacking a CC target after its current one dies. Whatever target you manually tell it to attack will automatically appear at the top of your pet's threat list. You shouldn't need to change its attitude in this example. This is only if you're not in assist mode, in which the game automatically makes sure your pet is attacking the same one you're hitting. In the time you're switching through the list of attitudes, none of the others will take any effect at all.
As far as your character's autoattacks, they always hit whatever you're currently targeting if it's already active. The only reason for
/startattack is to activate it.