Hi Andrei,
It looks like in your program you are writing values to the device, but you may be missing a command of some sort. Basically, it looks like your device is receiving these values, but does not know they are supposed to be values for current. So you might check your device or driver documentation and see if you need to pass a command first, and then send the values.
Also, I'm not sure which driver you are using, but Keithley does have a plug and play driver for LabVIEW available for this device on their website. National Instruments does not support or maintain this driver, but it might be another useful source of VIs that will work with your device. There is also a Release Note with more information about using the driver.
You can find them here:
http://www.keithley.com/support/keidoc_searchresult?keyword=2601&item_type=Software+Driver
I hope this helps!
Adam W
National Instruments