There are two ways to do this. Install the files from deb, or get the internet working, or both. I've grabbed my files for the raspberry pi from here in the past with great success. I then open a terminal and, in the same folder the file has been downloaded into, typed dpkg -i name.deb.
You must be in the same folder as the file - the easiest way to do this is to maneuver there in the file manager and right click then select 'open terminal here.'
I used files from this site to get my mobile phone working as either a source of internet, or as a wifi dongle (I have an android - but it should be the same for any phone that you can tether).
You will need a computer with the internet (presumably you have this to get the tarballs you mentioned.
Step 1: Download libpcap0.8, ppp and pppoeconf (sorry, I don't have enough reputation to post another direct link, but ppp and pppoeconf are in /main/p/ppp/ on the same server - I added the direct links in a comment below).
Step 2: Open LXTerminal in the same folder where you have the files saved and type dpkg -i libpcap then press the 'Tab' button (the rest of the name should auto-complete - if not, then you'll have to type it in full (or copy and paste the name).
Step 3: Do the same for ppp and pppoeconf.
Step 4: Open LXTerminal in the root folder and type:
sudo leafpad /root/interfaces.txt and add the following line:
iface usb0 inet dhcp
then save and quit.
Step 5: Enter ifup usb0 in the terminal
Step 6: Make sure your phone is connected, and tethering is switched on.
Step 7: Type sudo ppoeconf and follow the prompts
Step 8: Reboot.
You should now have networking (or at least I did). You can now apt-get install wicd which will give you better access to your networking.
Step 9: Open wicd, and from the drop down menu on the right select 'preferences' and replace eth0 with usb0 in the wired interface box. Tick the 'always show wired interface' box.
You can also turn wireless on and use your phone as a wireless dongle - just as handy, but without using your bandwidth.