You can do one of two things:
either use MC to generate an email for the logs from the Help->Logging page automatically (this needs an email client on your PC to do the email)
or click on the log in the same pane, save it somewhere and attach it to your forum post under additional options. It is considered polite to zip the file first to make the download easier/quicker especially if it is a big one. Txt files tend to zip quite nicely.
not sure if there's a preferred option? But either will get your log to the devs.
I guess it's nice to keep it in context and post it on the forum, because it will be apparent what it is about. Also a précis of the actions you took to generate the log/fault would be helpful in order to trace the log.
Note that if MC crashed, you'll need to send the previous log not the current one (because MC resets the log to a new one on startup). If you have a problem that is easily reproducible, then I would suggest a 'reset' of the log, reproduce the problem and then immediately send the log-to keep it as short and to the point as possible.
HTH
SBR