To determine if your kernal has AX.25 support, enter this command in a terminal:
grep ax25 /proc/kallsyms
If you get a result something like this:
c0510ddc r trans_net_ax25_table
c05127a0 r trans_net_ax25_param_table
then AX.25 support is built into the kernel. If you receive nothing, then you'll need to recompile the kernel with AX.25 support. (Most of the current versions of Ubuntu and Linux Mint have AX.25 support.)
For more information, see the HowTo:AX.25 article on the XASTIR wiki.