I'm running several Maintenance Plans on my SQL Server 2008 SP1 Standard installation. I'm running SQL Server Management Studio on both my development machine and a copy of it on the server. On either box, if I try and modify any of the existing Maintenance Plans or select "New Maintenance Plan...", nothing happens.
The only solution that I've seen for this was in this thread which suggested updating your install to include "Management Tools Complete" instead of Basic, but that option isn't available on my install screen. I'm guessing this is because I'm running Standard and not Enterprise. Note that this feature used to work so I know it's not a limitation of Standard Edition. I can't say for sure when it stopped working, but I believe the last time I used it was pre-SP1.