I don't see a listing for the Plus version on your account. If you are using the free version there isn't an update available to you at this time. The only update is the current beta of the Plus version (beyond the plus version it self of course). Once the next tier of the software is reached the free version will likely be updated.
If you use the Dock all the time I recomend updating since the software is inexepnsive for something used so often. There are a good deal of extras involved with the Plus version.
