Can't figure out Macs

This is my very first day working with a mac... mac mini that is.

I downloaded BlitzMax 1.18, updated to 1.24. Created a new file that said:

Graphics 640,480,0
EndGraphics

Clicked F5 to run. Gives me this:

Building untitled1
Compiling:untitled1.bmx
sh: line 1: as: command not found
Build Error: Failed to assemble /Applications/BlitzMax/tmp/.bmx/untitled1.bmx.gui.debug.macos.x86.s
Process complete


What am I doing wrong?

Problems compiling on OSX?

*sigh* I trully am a newbie... thank you.