Hi Hasan,
You need to make sure that you compile the flex application using the SeleniumFlexAPI.swc. If you are using Adobe Flex Builder 3 then this is easily achieved by right clicking and opening your application's project properties and then selecting Build/Compiler arguments (I can't remember the exact names as I'm not at work at the moment) and then entering the parameters in the additional compile arguments. It's safest to include the whole path name to the SeleniumFlexAPI.swc, so put it somewhere easy to find like C:\SeleniumFlexAPI.swc.
There are pre-compiled example projects (e.g. the selben/fruit project) available if compiling your own project is a problem and I'd recommend doing this to ensure your setup is correct.
Also be sure to include a link to the user-extensions.js in your Selenium IDE, by going to Options -> Options and browsing to the file.
All of the above is covered in the documentation so hopefully you've just missed something out. :-)
It may also be useful to post any error messages here, and use a testing framework such as JUnit or TestNG.
Hope this helps.