You haven't missed this anywhere! Believe me, the Logic manual is hazy at best about this feature and before your post I believed the Tap Tempo feature in Logic 8 was broken... well, I've since found that it isn't completely broken... but it is far from intuitive!
Here is one method to trying it out:
- Set up a software Instrument track.
- Open the Key Commands window and search for tap tempo
- assign it to a key, e.g. F7
- File > Preferences > Syncronization...
- Change the Sync mode to 'Manual' and make sure 'Auto Enable external Sync' is ticked!
- Options > Tempo > Tempo Interpreter
- Set the options as desired and increase the Max tempo change to 25bpm.
- Press play and use the F7 key to tap tempo!
Hopefully, the next version of Logic will allow for an easier workflow to do this... Would love to see a button in the Transport or in the Tempo Interpreter Window that can be a 'Tap Tempo Button'.
btw, if you use Dashboard and widgets, check out this bpm widget that will probably be easier to use: