Flex and Flash both based on ActionScript. Flash coded using AS3 and complied into SWF file. In Flex, actions are coded in Java and converted into AS3 and complied into SWF file. For Java developers, Flex offers a option to code in Java rather than learning AS3. Actually, building a Flex app is complex than Flash app, however, discussing that is out of scope of this post.
The following steps describe how to install and use FlexUISelenium in a few simple steps.
Download and install Selenium RC
Download Selenium Flex API, and rebuild your Flex application with SeleniumFlexAPI.swc
Download FlashSelenium and add to your test project
Download FlexUISelenium and add to your test project
Write and run your test cases against your Flex applications
Taken from http://code.google.com/p/flex-ui-selenium/
In the end there is no silver bullet for testing Flex/Flash apps in Selenium. Manual work is needed to add the ability of interaction in these apps.
PS: SauceLabs Flex Recorder claims that there is no need to recompile the Flex app in order to test. I haven't tried that so can't comment about that.
Thanks to my friend Vinod for helping me in this.