If you need to message a single creator there are two ways to do so.
For creators who are NOT in campaign with you, and have not applied to be in a campaign, you will need to message them via threads.
If they are in a campaign with you, there will be a messages tab next to each individual creator in the participants tab when you select that person.
If they have applied to a campaign, you will see a messages tab next to their name in the applications section.
You can also bulk message any creators IN a campaign, by checking the 'all creators' box in the participants tab.