I'm not sure what you mean by "changed the Flex compiler to use Flash Player version 10", but you need to ensure that the correct playerglobal.swc is included in your build.
For a Flex application, changing the "Require flash player version" setting on the Flex Compiler page to 10.0.0 or later will cause the version 10 playerglobal.swc to be included in your buid.
For a library project you will have to modify your build path to include the correct playerglobal.swc (Project Properties->Flex Library Build Path->Library-Path). You can find the FP9 and FP10 versions of playerglobal.swc here:
<path to sdk>\frameworks\libs\player\
Also, if you are not running version 3.0.2 of FlexBuilder then install the hotfix here:
And you must use version 3.2 or later of the Flex SDK.
Some of my problems appear to have been related to not running version 3.0.2 of FlexBuilder. I have now updated to this and can now access the FileReference methods I require in my Flex Project.
However, I still cannot access these methods within the Flex Library Project. The Library Path is pointing to version 3.4 of the Flex SDK, which I assume includes the correct playerglobal.swc.
Just had a go myself, simply switching playerglobal.swc does not appear to work for library projects. The compc option '-target-player' will probably need to be set to 10, but I can't see any way to do this in Flex Builder. Perhaps someone from Adobe can help, otherwise you may need to build with compc directly rather than FlexBuilder.
I changed the .actionScriptProperties file of the Flex Library project (I thought I had done so before) as described in the link and I can now access the Flash 10 methods.
Thank you very much for all your help.