I have started learning how to use the comping edit techniques described in the SONAR Plt videos.  After I have selected the appropriate takes, I have flattened the comps in a single wave form.  The take lane is soloed and locked, i.e., can not be edited further, e.g. put a fade on the end. 
 
Can the take lane be unlocked so I can add a fade?  If so how?  Otherwise, I have to delete the flatt4ened comp, add the fade on the last clip and re-flatten.

You should be able to unlock it in the clip properties. If you cannot do it that way, maybe you need to bounce the clip and render it, before anymore editing is done to it

Yup. It's actually locked on the clip level though... not in the lane.
 
Select the Clip > Right Click > Clip Lock then deslect "Lock Data"*
 
You can also do this in the Clip Inspector. Again select the Clip > Open the Inspector (I) > Click the Clip tab then look for the Lock entry. There is a drop down that allows you to lock/unlock the Dat/Position of the selected clip.
 
*Lock Position isn't engaged when flattening but it is useful to note that if you find you cannot move a clip check here first. Also I used to have a bad editing bug in X2 where random clips would get selected while I was editin other stuff so when I'd make my edits it would screw up other clips. Very frustrating but engaging the Lock Position (and Data) on all clips not currently being edited allowed me to edit without worrying. That fortunately has been fixed since X3 but if you find yourself accidentally doing stuff you don't want to (easy to do in a large project) this is a good solution.
 
Cheers.

BTW... I frequently edit my flattened comps. A lot of the time I'll create a slew of takes then create a few comps that sound good/have any mistakes editied out. I then move those into a new track and archive the old one (to save screenspace and conserve computer resources). From there I may unlock those clips and create new "uber" comps or comps that compliment each other well as doubles or panned. The first comping procedure is essentially to weed out the crap so it's like I'm the bestest studio guit player in the world providing multiple useable takes which can then be chosen from as a "producer". It's a pretty good workflow for me and I've been quite happen with the new editing features because of this.
 

The fastest/easiest way to unlock it is to select the clip and hit Ctrl+K.
Or you can assign "Lock Clip Position Data" to one of the buttons in the Custom module on the Control Bar beforehand, then select the clip and click that button. Unfortunately, "Lock Clip Position" and "Lock Clip Data" cannot be assigned to the buttons (since they aren't listed in the assignable commands). But they can be assigned to keyboard shortcuts.