If you had the farm before, chances are you may have selected to make it inactive.
1. Go to Add/Edit Farm
2. Find your farm in the list and deselect the Archive for this user check box
If you've never had the farm on your account before, and the farm was added by a co-worker or recently shared with you:
1. Sync first to make sure the data is pulled from the server
2. Go to Add/Edit Farm
3. Click on Sync Existing Farm to This Machine
4. Select the farm you'd like to add
5. Hit Continue to start syncing the farm to your machine
6. Once the Sync is complete, you will need to file sync for any maps or files to come through