I see a problem with the command you issued. The Desktop directory is generally capitalized. If you open a terminal from the start menu or a shortcut on the task bar it will generally open in your home directory. You would then need to:cd Desktop Perhaps though you made a typographical error in your listing. I think that is probably the case as I see another problem now that I look. The name of the file I have is PlaneShift_CBV0.3.020-x86.bin ,,, notice the -x86 before the .bin You might want to try chmod +x Pla* though that runs the risk of applying to more than one file. Another thing you might try is chmod +x Pla[TAB] invoking the Bash shells powerful tab completion function. Anyway make sure you have the right filename.
By the way the x86 refers to the computers bus architecture and indicates a 32 bit PC type processor; Intel, Amd or other compatible. The installer for 64 bit PCs has -x64 instead of -x86.