1. Get the package file linux568.zip and unzip it somewhere suitable.

2. Look on your system under /lib/modules.  There should be a directory
   there with a name that corresponds to the linux kernel you're running,
   and it should have a subdirectory called "misc".  On my system:
   
waverley:~$ ls -l /lib/modules/
total 1
drwxr-xr-x  10 root     root         1024 Mar 15 15:27 2.2.13/

waverley:~$ ls -l /lib/modules/2.2.14/
total 22
drwxr-xr-x   2 root     root         1024 Jan 17 17:30 block/
drwxr-xr-x   2 root     root         1024 Jan 17 17:30 fs/
drwxr-xr-x   2 root     root         1024 Mar 16 09:21 misc/ 

[listing snipped]

   If this stuff doesn't exist your kernel installation is messed up.
   Consult the Linux kernel HOWTO or any documentation that came
   with your Linux distribution.

3. Copy the file ltmodem.o (from linux568.zip) to the "misc" modules
   directory identified above.
   
4. Update the module dependencies:

/sbin/depmod -a   
   
5. Make the device entry for the ltmodem:

mknod /dev/ttyS14 c 62 78
rm -f /dev/modem
ln -s  /dev/ttyS14  /dev/modem
chgrp uucp /dev/ttyS14
chmod 666 /dev/ttyS14

6. Insert the module:

/sbin/insmod -f ltmodem

At this step you may get a complaint about discrepant kernel versions, but
see if the module has nonetheless installed OK:

/sbin/lsmod

You should see that ltmodem is now in place.

7. Dial up, have fun.

You may also want to look at 

   http://www.heby.de/computer/ltmodem

for further info.