If I'm viewing a user in the ACP, how do I go to their profile? I can't find a button/link anywhere.

I need to PM members who have made a purchase, or have subscribed and I can't find out how to get on their profile to do this from the ACP?

Do you mean to login to their profile?

Or to visit their profile? 

If you click in the user photo in the ACP profile, you’ll go to public side. 

You could also use this to send the PM from the ACP but it’s not compatible with 4.3:

@All Astronauts can you update it?

Functional version for 4.3+ is in Spacious ACP

I might get around to backporting it to the free one but it won't be for a few days at least.