You choose the target and it's forced. This is also a weird situation for D&D 4e to handle, since 4e, like other RPGs, is designed for players to cooperate, and the power's designers probably assumed you'd make the party's Defender mark the target to help them out, and not use it for PvP and mark someone squishy.
The power reads like this (the order of lines in a power is significant, because you do them one by one):
Target: One creature.
You pick a creature for your power, like normal.
Attack: Charisma vs. Reflex.
You roll to attack.
Hit: 1d8 + Charisma modifier damage, and the target is marked by an ally within 5 squares of you until the end of your next turn.
On a hit, you deal damage. The target (which is already chosen by this point) is marked by a nearby ally. Your choice again, and it just happens: it says "the target is marked", not "the target can be marked". It's forced.
Your ally may not know, but will be very much clued in
Nobody might realise it was you that did it, but the ally will understand they're now marking a monster, and the monster will understand they've been marked by your ally, because of this statement on PHB p57:
Whenever you affect a creature with a power, that creature knows exactly what you’ve done to it and what conditions you’ve imposed. For example, when a paladin uses divine challenge against an enemy, the enemy knows that it has been marked and that it will therefore take a penalty to attack rolls and some damage if it attacks anyone aside from the paladin.
They're going to think something's up. If they don't know who it is, consider further the flavour text for the power:
You conceal your arcane attack, tricking your foe into thinking the attack came from one of your allies.
That's only what the attack might look like (flavour text is entirely replaceable - see PHB1, p54, under Flavour Text) but if an ally sees a signature arcane attack suddenly appearing from in front of them, they're sure going to have some suspicions.
This is not going to guarantee enemies attack weaker party members
A mark only means the monster will get a -2 to attack for attacks not involving its marker, which is hardly the end of the world for it unless it absolutely must hit something. Being marked doesn't mean it will attack your ally, it just means it's slightly less advantageous than before to attack anyone else - they might happily continue to wail on someone that isn't that ally, and maybe even you!