We did the following steps for displaying mashup(given in menu) in contained mashup.
1. Created a new Menu "TestMenu". In Menu definition we gave Type as "Mashup", Link as some sample mashup "SampleMashup1" Target as "Replace page"
2. Added a new Master "TestMaster".Dragged and dropped a menu and selected it's menu as "TestMenu"
3. Opened "SampleMashup1" and in settings gave master as "TestMaster".
4. Executed Testmaster. On clicking menu it replaces the whole page to "SampleMashup1" instead of opening it in contained Mashup.
Please let me know if i am missing any settings here.
There is a way to have the menu within a mashup and load the mashup selection in the contained mashup area, but unfortunately there is a bug in this functionality. This has been logged with development and will be resolved in the release of 5.0 SP1. Once it is released, you can set the "Drive Page Mashup or Bound to Mashup Parameter" option in the menu widget properties to +Bound to Mashup Parameter +and bind the Mashup parameter of the menu widget to the name input of the contained mashup.