Just to ask you a somehow silly question - and I apologize if it is: is fastboot installed on you pc - windows, linux, or whatever OS you're using? I ask you because it sounds as if your OS couldn't find the command anywhere.
Just in case, if you downloaded fastboot and adb from somewhere else and not the official android sdk platform, go to the directory/folder where you downloaded fastboot.exe or adb.exe (I'm not sure about names since I run Linux) and try everything that rhen told you from there.
I think Cyda recommended not using internal storage as default for downloads... when in Link2int mod...You should do use your external SD card instead. See the first posts in link2int for info from Cyda on this.