Show the error message, after I accidentally clicked "Yes" on "don't show error again" I was trying to alter an existing Maintenance Plan, but received an error on doing so. Before actually reading it I clicked it away, too late realizing that the text said "Do you wish to hide this error message in the future?"

I tried logging out/logging in, resetting my settings, browsing through Tools > Options but the error won't show again. How can I reset my earlier decision and show the error message?

It's Microsoft SQL Server Management Studio version 11.0.2100.60.
You could try shutting down all instances of SSMS and deleting the file SqlStudio.bin, which works for clearing out things like the MRU list for server connections. Just note that this will also wipe out other preferences, so you may want to export certain settings (like if you've customized fonts and colors, startup options, etc).

Also, why is SSMS RTM? You should at the very least apply Service Pack 2, if not use a newer version altogether.