ATF doesn't shipped an Ajax runtime, but almost any Ajax Runtime can be configured to be used in ATF.
After a runtime is configure, it will show up in ATF wizards such as selecting the runtime when creating a project.
To configure and Ajax Runtime, download the runtime and unzip/tar the toolkit into a directory.
Open the ATF Preference page for Configuring AJAX Runtimes, Select Window > Preferences... > ATF > AJAX Toolkits
Click the "Add.." button to add a runtime.
Press the "New..." button to define a Ajax Runtime type. Note: For historical reason Ajax Runtime Types for Dojo, Rico, and Scriptaculous have been predefined.
Fill in the "New AJAX Toolkit Type" information.
After the Runtime Type has been defined, the type will show up in the drop down box.
Fill in the "New AJAX Runtime Instance"