From the Admin > Agents (1) > Access rights (2) you can set which campaigns your agents have access to.
Please note: This should not be confused with the "Agent and contact list selections" in the campaign settings. That one only sets the campaign that pops up when the agent logs in.
From the table you'll find all campaigns on the vertical bar and all the available agents on the horizontal bar. When an agent is given access to a campaign, (s)he can select the campaign from the Agent interface on Call tab via the dropdown menu.
As an admin you can change the rights by clicking inside the table, where you can set access rights per agent. A green check-mark represents the agent having access, a red check-mark (5) (only available via Full access rights) represents the agent being blocked from access completely.
"Full access rights" (3) gives an agent access to all campaigns, given that under the Campaign settings > Optional settings > "The campaign is selectable by agents with full access rights" is checked.
If full access is not relevant, you can select multiple campaigns per agent (4).