The Mandy is built on RedHat and it uses rpm as package management database, so whenever you upgrade your packages or install new ones using rpm the db is updated. Now, to start off take a look at
rpmfind . There are a lot of stuff for Mandy and other distros that use rpm.
After you d/l'd an rpm package (filename.rpm) you can install it, I guess for now just open up the file manager window (in KDE it is konqueror) and click on the package, it will ask you for the root password during the process, you should enter it.
There are also packages known as tarballs (usually filename.tar.gz) they are installed differently. After downloading them unpack the archive with gnozip or archiver found under K->applications->archiving I guess, after you unpacked them read the INSTALL or README files on how to install the software, it imvolves the use of console (like DOS prompt in windows)
Check out these sites:
www.freshmeat.net
www.slashdot.org
for more info,
www.linuxnewbie.org
is a very good source for a newbie