What applications are you trying to install? are you building them from source? A good slackware resource is linuxpackages.org, where you can find current .tgz files. Onceyou have the package, get a root terminal, and use
installpkg <package filename>
alternatively, you can use
to get a curses-based tool to view, add, and remove packages.
The MBR should be fine for LILO. Versions of windows after win2k came out are selfish aobut the MBR, though. Be sure to make the rescue floppies.