The Total Codex was just a box set including M1, M2, and a whole stack of 3rd party plugs that were popular in the early days of M2. All these installs are 1.3, but can be updated to the latest through this website
Once you get & install the total codex versions of TFL & Myth II you just need to download the update for Myth II 1.5 and hold on a few weeks for the 1.5 update for TFL and you will be set to play.
'a few weeks for the 1.5 updater for TFL'? It's a long time since I've seen concrete evidence that TFL 1.5 is not too far away, but I'm not a beta tester so I can't be trusted....
PlayMyth.net only hosts downloads of TFL 1.4.5 for Mac OS X and 1.4.0 for PC. I can't play with my PC buddies cuz 1.4 isn't compatible with 1.4.5. =(
TFL 1.5 has been a very slow work in progress. At this point it is mainly waiting for some possible enhancements from Myth II 1.5.1 and I have a single bug to look into also.