It sounds like you are running through the Flex 3 exercises while trying to use Flash Builder 4. This will not work, since Spark components were not introduced until Flash Buiilder 4. If you set the Flex compiler within Project Properties to use a specific SDK of Flex 3.5, then you should not get an error.
Hope this helps.
You are right. I am using FlashBuilder 4.0 and was trying out the video tutorials that belong to 3.0. But could you help me to populate a Simple Combobox in FlashBuilder 4.0? Also the mx components are supposed to be compatible with the higher version of FB4.0?
If you must use Flex 3 but want to use Flash Builder 4, you can choose the 3.5 SDK when you create a project in Flash Builder, and you will be building a Flex 3 project and all the old videos should work for you.
If you want to use Flex 4 and Flash Builder 4, then I suggest you learn from the Flex 4 and Flash Builder 4 videos (http://www.adobe.com/devnet/flex/videotraining/). That way there won't be any confusion between the old and new versions.
You can download the solution files for any of the exercises. The one called "Adding data to your application" will help you populate a DropDownList control. HTHs.
Thanks a lot. It really helps. I was unaware on the Flashbuilder 4 Video tutorial availability.