You should take a look at the release notes PDF. It will list the steps to build a player in either Flex or Flash CS4/5. You're going to likely use the OSMF.swc library and create a MediaPlayerSprite object in your project. You don't need to figure out how to build that .as flle you mentioned. See here:
Whoops, didn't read your post correctly. Yes, you will need Flex Builder for that sample project. Most of the sample projects require Flex Builder.
>> Note: Flex 3.2 or higher is recommended to run the Flex sample applications.
Are you acquainted with the OSMFExamplePlayer that is compiled and hosted at this link:
Regarding compiling, the OSMF framework itself is straight ActionScript 3. However, in apps\samples some samples use Flex components.
Any OSMF samples that are not importing Flex classes (mx.*) can be compiled in Flash Pro.