If you want ePub or LRF, you can now use the Calbre GUI to do the conversion from MOBI.
If you want to "explode" the MOBI to HTML for later conversion to some other format, then Calibre's command line tool mobi2oeb (described here) may be your best option. It is very easy to use, from the command shell issue the command:
mobi2oeb -o ebookname ebookname.mobi
where "ebookname" is the filename and the "-o" option tells mobi2oeb to put the result in a subdirectory called ebookname. If you already have the MOBI in its own subdirectory you can leave off the "-o ebookname". Your ebook might also have the extension .prc, this is perfectly ok (just use .prc in place of .mobi).
For information about how to get a command prompt, see Command Prompt Vista/XP/Mac
. You would then navigate to the directory containing the MOBI ebook using the cd (change directory) command. I generally copy the full directory path from a window open on the directory and paste it into the command window after the cd.