Ok this is from memory here (at work, not a mac in sight

).
On the PC you need to go to c: \document and settings\username\application data\thunderbird\profiles\ in this directory you'll have another directory with a fairly random name (xxxxxx.default) and there is a file (I think it's called profile.dat) you need to copy the profile directory (xxxxxx.default) to a USB Key or something.
On the mac, open finder, on left hand side there should be a folder called Library, in this folder you should find a folder called Thunderbird followed by a folder called Profiles. Copy the profile driectory (xxxxxx.default) from your USB key to the profiles driectory on the mac.
Now this is from memory - I think there is a file in the profiles directory called something like profiles.dat - you need to open this file, you should see the name of a profile to load, you need to change the name from the one that is there to the name of the profile directory moved from the PC e.g.
[profile]
yyyyyyy.default
change to -> xxxxxx.default