This is going to be "down & dirty" because I am surfing the 'net with Win98SE/AOL and can't get to MDK 9.0 on another drive.
I posted an answer to a question similar to yours on another Forum and I'm going to try a "cut & paste" so this may not make "perfect sense" !!
PASTE JOB STARTS HERE
All I can tell you is my experience, I have an INTERNAL modem that "appears to be working" even though I don't have a Linux ISP. [still using AOL].
With Mandrake 9.0 and the KDE desktop:
I click on KDE menu icon and go up to "What to do".
Over to sub-menu "Use Internet".
Click on "Connect to the internet".
A kppp window appears.
I click Setup button.
The kppp configuration window appears.
I click on Modem tab, then Query Modem button.
The ATI Query window appears and under ATI 3 my modem is listed.
"U.S. Robotics 56K FAX INT V5.22.45"
If I click the Modem Commands tab, the usual modem data appears.
END OF PASTE JOB
If you can follow the steps from KDE menu, the MDK Control Center steps are pretty self-explainatory.
If somebody else doesn't post with "better help" I'll try to get back when I have my copy of MDK 9.0 up & running.