Linux is unique in operating system in the fact that support for ham radio ax25 packet is actually built into the operating system, this make the handling of packet radio very efficient.
However since the majority of Linux users don't want to run packet radio, this feature is turned off by default, and if you want to handle radio ports you need to turn it on
(You don't need to do this if you are only using the Internet)
I cannot give you exact step by step instructions on how to do this next bit, because it depends on the version of Linux you are using and also if you are using a GUI (graphical user interface) or not
Basically you have to rebuild the kernel that Linux loads when it starts up, you usually have several of these, and your system is set to boot to one of them, often with the option to select a different one at boot time, so if your first attempt at creating a new kernel ends in disaster it doesn't matter as you simply select the last good one at boot time.
In order to include the Ax25 feature you therefore need to re-build the kernel, so you will need to obtain the source code, this is an ideal oportunity to ensure you are up to date with your kernel by downloading the very latest, or you can obtain a ready built one with ax25 built in, by searching around the Internet.
You can download the kernel source code at www.kernel.org and you can get detailed instructions on how to rebuid it at http://www.linuxdocs.org/HOWTOs/Kernel-HOWTO.html you should also search for help on this on your own Linux machine, or on your Linux versions web site, as many releases of Linux now have a much simplified graphical interface that guides you through the whole process.
Basically you are asked a series of questions about features you want in your new kernel, you can accept the default suggestions in most cases, but you can also make your kernel faster and better at this time by omitting support for things you don't have on your computer.
You then move your newly built kernel into the boot folder and set it to boot up, some Linux versions do this automatically when you build a new kernel, some do not.
Check your new kernel works properly, then move to the next stage, adding the Interface library between your future BBS and the new Linux kernel.