Possible duplicate: Android multidex support library using eclipse
It looks like ADT's ant-tasks project is no longer maintained (it resides under 'legacy' folder).
So if you can't migrate to Gradle, you can edit the DexExecTask manually. You'll have of course to rebuild the project locally..[Edit - 10/25/2014] maven-android-plugin does support multi-dex.
However, it currently has a small issue: secondary dex files placed in wrong location. This pull request strives to fix that, so stay tuned!(Fixed in 4.0.0)
I would suggest migrating to Android Studio.