You may have to install kernel source code in stx in order to compile the modem module. Alternatively you can do a minimal Slackware install and compile the window manager the comes with stx.
After spending alot of time installing source code so I could compile a wireless driver in zenwalk (another slackware derivative) the file system became corrupt and I was unable to repair it. The second time around I installed a minimal Slackware 10.2 install and compiled my own XFCE desktop and it has been running great on my Via C3 powered laptop
STX uses the EDE desktop
http://ede.sourceforge.net/page/