Fixed this problem occured in Flex3 beta2 with svn.
1.copy the missing package(org.eclipse.jdt_3.2.1.r321_v20060905-R4CM1Znkvre9wC-.zip) to \Program Files\Adobe\Flex Builder 3\features\ ;
2.with site-1.0.5.zip;
Finally ,fixed this problem.
Other ways: FLEX->help->soft updatas->search new >>;
Europa Discovery Site->Next;
Auto Mirror filter;
Java Development->Eclipse Java Development Tools;
Selected Remote site:
