We are no longer using Selenium IDE for Telerik UI for ASP.NET AJAX controls testing. Currently, we are using Telerik Test Studio
as well as the free Telerik Test Studio Testing Framework
for test automation.
You may find information on how to automate with Selenium IDE in previously published blog and forum posts. I suggest you to check the Selenium Scripts for RadControls, Firebug and XPath Support blog post
and the following forum thread concerning RadComboBox testing with Selenium
I would recommend you, however, to try the Testing Framework and Test Studio products, since they provide a rich API that expose numerous properties and methods to help you easily build non-brittle, maintainable functional tests.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.