To re-activate a client go to Clients --> Clients.
There you'll see the list of the active clients by default (the deactivated clients won't be displayed on this list), but you can perform a search of clients who have been unsubscribed selecting Uns. on the Status drop-down menu (to see this option and a few more you have to previously click on the green arrow next to the magnifying glass icon)
Once you selected the status you'll get the list of unsubscribed (or deactivated) clients. Now you just need to press the green arrow at the end of the line to re-activate the client.
You can also re-activate a client from inside his/her file. On the previous list press instead of the green arrow, and you'll enter the client's file.
At the bottom of the file in the General section you'll see a button called Reactivate. Go ahead and click on it and the result will be the same as with the other option, the client will be active again.