Most probably a missing import or a missing library in your build path.
You may need to add the library SWC file to your Flex project libs folder, or if you do not wish to place it in your project, from the Flex Builder menubar:
Project - Properties - Flex Build Path
Click the "Library path" tab, then "Add SWC..." or "Add SWC Folder...".
If this post answers your question or helps, please mark it as such.
Greg Lafrance - Flex 2 and 3 ACE certified
Flex / AIR Development, Training, and Support Services