Make sure you copy the -entire- launch line into a single line.
You are seeing that error as the line got broken into two parts; the first part starting with "/usr/bin/java", and the second with "jagexappletviewer".
I suggest sticking it in a text file first(name it "runescape.sh"); makes things easier.
Then you can just type:
"cd *directory you put runescape.sh in*"
"chmod +x runescape.sh"
"./runescape.sh"