First you have to have a tree in Ancestry.com.Log on to Ancestry (you can do it thru FTM in the Web dash board on the plan workspace, or directly).Open FTM 2012 and go to New Tree tab on plan workspace and click download a tree from Ancestry.You should see a list of your Ancestry.com trees.Pick one and click the request download button in the right most col.Follow the instructions.Hopefully after a few minutes you will have your Ancestry tree in FTM with the sync option working.
You can choose manual or auto sync.Instructions should be under the setting up a tree section of the manual (paper or digital) that came with FTM.There are other sync options as well, i.e. build a new tree and sync as you go...see manual or call help desk.see below for number.
In my case FTM would not complete the download process for my tree.Programmers are still working on the problem.Best I can tell(based on what I have been told), there may be data in my tree that acts like programming code and causes the process to fail.As far as I can tell, I didn't add any propgramming code, just photos, text and Word documents, pdf documents and stuff linked from Ancestry.com searches.
Sync does appear to work, I tested the process by downloading and syncing a small tree (about 90 names) and made changes in both Ancestry and FTM and had the changes sync to the other tree.I added, changed and deleted data.Problem is I reported the problem in September or early Oct and it is now almost Dec and no resolution for my problem.
Good Luck, the help number for FTM is 1-800-262-3787.Help desk is very responsive and knowledgable.They just haven't been able, so far, to fix my problem....