Alternatively, simply use
CALIBRE to manage your ebook collection. You can either pre-convert EPUB to MOBI or let it do that on the fly when you send an EPUB to the Kindle, (which doesn't need any jailbreaks).
As the eink Kindles have quite primitive web browsers with restricted download capabilities, you cant download an EPUB directly, so side-loading your books is usually the best option.
However I do have my K3 jailbroken for other reasons with three different program launchers installed. KUAL for convenience, KITE for the ability to put script links into any Kindle collection, and Launchpad for quick access to MYTS and a few other utilities I may want to run from any screen.