You need to quote the argument...
Messagebox takes one argument, but if you put:
MessageBox This is my message
It's actually 4 arguments...
MessageBox "This is my message"
Is one argument, and MessageBox will work. Environment variables and "specials" ($eol$, $lt$, and $gt$) are converted AFTER the ActionScript interpreter has read the argument:
MessageBox "This is line one$eol$This is line two"
Is read as:
This is line one
This is line two
The "EOL" or End-of-line "special" is replaced by a carraige-return line-feed sequence and the quotes, of course, are not passed to the MessageBox command.