Hi Tom,
Can you please describe the bug?
Is it in the Tor Trac with a bug number?
In general, if you start in the tor directory of a source distribution:
(I'll use gitweb links, but you can use git and clone a local copy)
tor/src/or
Contains the tor onion routing-specific source code
Your bug is likely to be here
tor/src/common
Contains the common facilities used by the tor/src/or code
Your bug is also likely to be here
tor/src/ext
tor/src/trunnel
tor/src/win32
Contain external libraries and platform-specific source
Your bug is probably not here, or in any of the other directories