just checked the code and have got mainloop defined - don't why it didn't copy over.
Code:
 ' Name        : Test44.pbp
' Compiler    : PICBASIC PRO Compiler 3.07
' Assembler   : PM
 'Button press turns on LED  

mainloop:

    If PB = 0 Then ' If button pressed...
     low LED      ' Turn on LED
   Else
     high LED     ' Turn off LED
   Endif

   Goto mainloop  ' Do it forever

   End
I am using PBP3.071 and and I guess MPASM if that is the default installed.