I thought hints did work in MP.
They do. However, the "hint" command has to be executed on all client machines (IIRC). I'm guessing you're doing the "hint" in either a script running on the server or only on one client.
Put the hint line in a trigger and have that trigger activate somehow.
That "somehow" could be all kinds of "somehows", like a vehicle entering the triggers activation area or by having the trigger wait for a public variable to become true. The latter would require you to initialize that
variable (let's call it "hintplayers") in for example the init.sqs.
Then, in a script running somewhere, preferably on the server, when the time is right, there's a
hintplayers=true
publicVariable "hintplayers"
If you tell us a bit more about when/why the players need that hint, we can perhaps give you better advice what to do.