Just tried updating a colleagues instance and it failed with the same error:
An error occurred during the org.eclipse.equinox.internal.p2.engine.phases.CheckTrust phase.
session context was:(profile=epp.package.cpp, phase=org.eclipse.equinox.internal.p2.engine.phases.CheckTrust, operand=, action=).
Error reading signed content.
The file "D:\Development\Applications\eclipse-cpp-kepler-SR2-win32\plugins\com.slickedit.core_4.3.0" does not exist
After a bit of trial and error I've found how to successfully update it. Manually delete anything in your Eclipse installation that has slickedit in the filename (a bunch of folders in "features", a bunch of folders in "plugins", and a file in "configuration/org.eclipse.osgi/manifests"). Start Eclipse and uninstall SlickEdit. This will restart Eclipse - when its back up reinstall SlickEdit.
There seems to be a couple of issues here - first the update doesn't work. I presume this is because the version number isn't changing (just the build date). Second, the uninstall doesn't clean up properly, it leaves a bunch of SlickEdit files lying around. If you try to simply uninstall then reinstall, the reinstall seems to fail (it wont update some of the files), then anything I did from this point seemed to generate the error above.