Chntpw is a windows SAM password recovery utility. In BT4 R2 it can be found in Kmenu>Backtrack>Privilege Escalation>PasswordAttacks. There are two minor issues with it on BT4R2:

In the kmenu entry, the path is wrong, it is /pentestpasswords/chntpw/chntpw instead of /pentest/passwords/chntpw/chntpw. Just a missing forward-slash between pentest and password.. probably just a typo.
It is also not in root's PATH variable.

It is very easy to fix both of these errors:
for the kmenu typo just right click on the entry, then "Edit Item" and add the missing "/" in the "Command:" field.
for the PATH variable problem just add it manually with
These are not BIG problems but when I use chntpw I'm usually booting off the liveUSB on a windows computer, and I have to fix these every time (at least the PATH thing)

I hope this is fixed in the next release