By double-clicking a file named "build.xml". If your build script is named something else, then you need to map the filename to the Ant editor under "Window > Preferences > Workbench > File Associations".
Do you get syntax highlighting and a correct outline?
If double clicking the file doesn't work, right click it and choose "open with -> ant editor"...
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus