Set the handset speed to 230400
for H100: menu > 7 > 3 > select 230400
for S200: menu > 6 > 2 > select 230400

download pppusb002.zip archive from
http://www.zappmobile.ro/data/store/drivers/pppusb002.zip
copy the files from archive (ppp folder) to your linux /etc/ppp

source files:
zapp_linux_ppp\ppp\chap-secrets
zapp_linux_ppp\ppp\chat-sprintpcs
zapp_linux_ppp\ppp\options
zapp_linux_ppp\ppp\dialusb

Note: overwrite the files (you may want to make a backup of the old files)

edit /etc/resolv.conf
and enter the name server from Zapp Network:
nameserver 172.16.253.242

to bring the PPP connection up:
go to /etc/ppp folder and run
./dialusb

to shut down the PPP connection:
killall pppd