double dot problem

I have problem when trying to change duration of the note to double dotted. There is no such command in palettes, and I try to add a special shortcut using Preferences->Shortcuts. I try with different shortcuts but I have the same problem. One manifestation is that I get strange "notation" - for example if the note is a quarter, I get the same quarter note, but followed with strange rests. If I click on the undo button, I get the double dotted quarter note! The other manifestation is: it doesn't do anything! In this case I can repeat the shortcut more then once, but with no reaction.