If you subscribe on Steam, it should download the necessary files to your game install. Then you need to start up Solasta (or restart if it's already running) and choose it from the available campaigns when you start. Characters have to run completely through a campaign before they can visit a new campaign. Also, remember that if you take characters to a non-official campaign you can't take them back into an official campaign.

If you hit NEW ADVENTURE and press that "Change Campaign" button on the left of the screen you should see your workshop subscriptions there.

Originally posted by Strad:

If you subscribe on Steam, it should download the necessary files to your game install. Then you need to start up Solasta (or restart if it's already running) and choose it from the available campaigns when you start. Characters have to run completely through a campaign before they can visit a new campaign. Also, remember that if you take characters to a non-official campaign you can't take them back into an official campaign.

If you hit NEW ADVENTURE and press that "Change Campaign" button on the left of the screen you should see your workshop subscriptions there.

Correction: You can use any character from your pool to any campaign (may be it official or custom). Once you export characters from custom campaign and retain their acquired level, only then you cannot access official campaigns with those characters. The reason is the officials start from level 1 to any current max level, so you need to have level 1 characters. You may create from scratch a copy of the characters to use elsewhere, or use the community expansion mod to level the already established characters down.