You need to mention the default target in the project tag of your build script so that when you try to run the ant script it'll take the target mentioned in the default attribute and try to run it.
<project name="B2G" default="build" basedir=".">
If you haven't mentioned this attribute you have to use
ant [target name]
kind of syntax to run the script.
This applies to any editor you are using or you execute from command prompt.
In eclipse you can configure which target you want to execute in the build script without specifying the default attribute using external tools
option in the run
Hope its helpful.