Step 1: Add the build file to your ant view
Click the add button and select the build.xml file from the project. You can also drag the build.xml file onto the ant view (To add the ant view to your perpective, go to Window > Show View > Other…)
Step 2: Run as Ant Build
In your ant view, right click the newly added build file and select Run As > Ant Build… In the dialog window, select the ‘Hide internal targets not selected for execution’ option in the Targets tab. You can rename the builder in the box at the top of the window if you wish (I usually remove the trailing ‘build.xml’. Click Apply and Close.
Step 3: Setup automatic build
Right click on the project, select Properties, then Builders. Click Import. Before closing the screen, highlight the imported build and select edit. Go to the Targets tab select clean for ‘After a clean’ and deploy.local for ‘Auto build’. Apply the changes and close.
Step 4: Enable Auto Build
Go to Project > Build Automatically
source: http://www.simonwhatley.co.uk/using-ant-with-eclipse