LAST EDITED ON 09-23-09 AT 08:33 AM (EST)Check the CBS website as the application will be posted when they are open.
Usually they shoot the Spring edition during the fall edition so applications for spring may already be over, or if they aren't would be very soon.
For the fall edition it tends to be later in the spring season because they shoot that during the summer, not while the spring edition is airing.
ETA: http://www.cbs.com/casting/index.php?show=1&0.7449599941820261 is the direct link to the casting for TAR, looks like it may be too late or getting too late for the spring edition, so you might have to wait, but they have the forms to download and eligibility requirements and all that up.