Below is a general description of what you need to do depending on various factors, I am not going through a step-by-step guide since there are good ones already out there. There is a comprehensive guide HERE on full set up.
There is another page HERE with useful info also.
On another note, when you say you have the RPi as a control unit, let's make it clear now that the ASICMiner Cube is a standalone miner, it does not need a "control unit" such as say a bank of USB Block Erupters or Antminer U1's. You could hook it straight up to your router with a network cable and begin setting it up straight away. You need your home network to be in the IP range of 192.168.1.* since the default web control panel for the cube once powered on and connected is 192.168.1.254:8000 (see above guides for more info if needed).
What you need to do next will largely depend on which pool you wish to mine on. The cube uses the outdated "Getwork" protocol which many pools no loner support, in favour of the better Stratum protocol. If the pool that you wish to use still supports Getwork then you don't need the RPi at all. Just get the URL, port, worker details from your pool account and input into your cube.
Where your RPi will come into the picture is if you wish to mine on a pool that is using the stratum protocol and does not support Getwork. The RPi will act as a simple proxy that you can leave on 24/7. You could set up this proxy on your desktop or laptop, but that means you would have to leave it on all the time and it would use a lot more power than your tiny RPi.
So first off you can download and set up the Stratum Mining Proxy onto your RPi from HERE following the link for "source package for Linux". It's a very simple case of just cloning the git repo to your RPi and then running the Python script with any required parameters (also see above guides for explanations on this).
Its a good idea to run the proxy within Screen on your Pi so you can detach and reattach later if you want. It also keeps it running when you close the SSH session. Once you have set up the RPi with the proxy running you would then log onto your cubes web control panel (192.168.1.254:8000) and then instead of pointing the config directly to a pool, you point it to the IP address of your RPi running the proxy on your local network.
Here is a screen shot of what your config should look like:
https://dl.dropboxusercontent.com/u/67953101/cube.jpg
IP = the IP of the cube (192.168.1.254 by default)
Mask = your local network mask
Gateway = Your routers IP
Web Port = the port you can connect to your cube through web panel (8000 by default)
Primary DNS = Your routers IP
Secondary DNS = not really needed
(The cube needs at least two pools to mine, so you must input the below twice, comma separated.)
Pool ports = ports that you assigned in the stratum mining proxy
Pool addresses = in this case the local IP address of your RPi on your network (In my case my RPi is 192.168.1.3, change it to whatever yours is)
Miners user:pass = the details of your worker:pass details from your pool account
Once you have input your config, hit Update/Restart and after a short time your cube should start mining, (hit the Refresh button in the cubes control panel to refresh the page not the browser button.)
It's a good idea to have the SSH session alive to your RPi while this is happening with the proxy open in Screen. Once your cube connects you will see it in the proxy output and if all good you will see it start to acknowledge accepted shares etc.
I hope that gives you a general idea of what you should be doing, as I said there are detailed guides which if you follow step by step you shouldn't really go wrong.
Good luck
Oli.