Jochen,
If you refer to keithley website, it says there that "
National Instruments' Measurement and Automation Explorer (MAX) will not detect the presence of non-NI hardware. It is not necessary for MAX to detect the card for successful operation of the Keithley GPIB cards from LabVIEW."
ftp://ftp.cec488.com/members/cec/488/lvsupp.exe
The IEEE 488 VI's are located in the functions palette >>Instrument I/O>> GPIB 488 and should work with your keithley(according to them).
here are are a few links in the NI-Developper Zone that should help you out:
http://zone.ni.com/devzone/devzone.nsf/webproducts/a95c253dfb5fdb0986256786000be3c3?opendocument
Also check out your LabVIEW examples in Help>>examples>>I/O interfaces>>GPIB.
Regards
Cyril Bouton
Applicat
ions Engineer
National Instruments
Cyril Bouton
Active LabVIEW Developper