The package is not available in Composite C1 4.2 Beta. Please check.Just tried upgrading a clean 4.2 Beta 2 to 4.2, and there were no problem.
Check what's the version of /bin/Composite.dll you have. The package should show up in the list when Composite.dll has a version within interval [4.2.0.0, .., 4.2.5249.32245]
That means, if you replaced the file with 4.2 version, or your custom build of Composite.dll, it will disappear from the list of available packages.
Replacing the old Composite.dll should make it available again.