At a guess, it appears that you are in your user folder and the device tool is on the desktop.
Originally Posted by Laniadoje
So, from C:\users\laniado>
I'm not a command line expert and the above is a guess,
C:\users\laniado\desktop>java -jar devicetool.jar
not something I've actually tested, but I think that fuddy78's example is navigating that path from C:, whereas you are entering the full path when you are already part way there. I guess the terminal opened at C:\users\laniado\
Update: I tried running a webOS doctor from it's own folder using the full path and it failed. Running just the command java -jar webosdoctor worked, so it seems that path IS relative to location.
cd means 'Change Directory' (directory=folder). Where ever you are, you can type 'dir' to list the contents of the directory. Items on the listing that are folders or directories (listed as <DIR>) can be opened by 'cd folder-name', then you are in that folder and can 'dir' again to see the contents. To go up a level of the folder tree, type 'cd..'
So I think you typed the command right, but needed to have cd..'ed a couple of times to start at C: or cd to the desktop (if that's where devicetool is) and run it directly from there - full path not needed, because you are already there.