From the load profile screen, click on the profile you want, and click edit

Check any mods that you want active in "Active Mods" Or uncheck any mods that you do not want active.

Then click apply, and enjoy your mod.

And don't forget, Da Name's Rick James
Dis old: (You don't know how much trouble I had to go through to get this post up, that's the last time I use my iPad for this site, looking toward that Surface PC from Microsoft instead. I have had it with my iPad)

7/15/2015
GT-Mike: 1.19
Here's a tutorial on how to activate a mod in new mod manager.
Step 1. Open mod manager (for the purpose of this tutorail I filtered displayed mods to "trucks" only, it works cause my P mod is updated to 1.19 already, for now filters don't work on existing mods)

Step 2. Find an installed mod you want to activate and click it - it should highlight like this:

Step 3. Click on the single right arrow to activate the mod

Step 4. Your mod appears in "Active mods" list and mod indicator in mod browser changes to green

Step 5. (optional) You may use buttons below "Active mods" list to change mod priority (load order)

Centrino2's Top Tip's:
Clicking on "Increase Priority" or "Decrease Priority" moves the selected mod Up or Down one slot.
Clicking on the "Up Arrow" or "Down Arrow" next to the Priority Buttons will send the selected mod to the Top or to the Bottom of the priority list.