You may find out that Xfoil is not in the Ubuntu repositories. For compiling the source package, you need g77 compiler, but that is not included to Ubuntu Intrepid repositories right now and getting it to work from the source seems to be a lot of trouble. Here is what I found, after some digging, a ready made package which installed on Ubuntu Intrepid fine:
Download the file and install with
sudo dpkg -i xfoil_6.97-1_i386.deb
If some of the dependending libraries are missing, just install those from Intrepid repository and it works fine without problems. Have fun!
For more reading about XFoil, please see:
Xfoil tutorial with illustrations
Terrabreak.org XFoil tutorial
More on Xfoil at mh-aerotools