What usually works is to simply google the package name and add maemo to your search string. Id est: searching for libc6 and maemo will return an address like maemo.org/packages/view/libc6/.
Provided you have the correct repo's enabled in HAM or FAPMan it should work there too. But the default app manager (HAM) is so terribly slow I prefer the command line above anything else.
Edit: Typo's (Not used to my new steelseries keyboard just yet)
You seem to miss the basic understanding in installing applications.
The additional libraries are hidden in HildonApplicationManager (HAM) totally and hidden in FasterApplicationManager (FAM) by default. You could make them visible in FAM in the categories view, but to install any application will pull the dependency libraries automatically in both managing apps.
Because pure command-line tool apt-get will pull in those libraries, which those manager use in the background.
If you try to manually install packages, you must not use any file manager by clicking(tapping) at the icon of the downloaded .deb package, but use the X-Terminal application, gain root rights via some packages, that allow you to, and use dpkg command-line tool.
Long story(sorry for the bla,bla),short: Your repository setting seem to be incorrect. You could post it here for others to check. Please paste the output of the terminal command
