checking for X... configure: error: Can't find X includes. Please check your installation and add the correct paths!
You need to install the x11 devel packages, you can find these on the install cd's
could not find any kernel sources in /usr/src
you have either not installed your kernel sources
or your kernel sources are installed in another place then
You have to install the kernel headers, they're also on the install cd's but you have to select them to be installed, theyr'e not by default.
Sorry but I don't know the answer to your third question, perhaps if you included what type of modem you own and which driver you installed someone else could find something?