TriggerParameterPath is used for passing a property in the event arguments of the event to the command.  SelectedItems is not a property of SelectedItemsChangedEventArgs so that won't work.  For a list of properties in SelectedItemsChangedEventArgs, please see the following: http://help.infragistics.com/doc/WPF/2014.1/CLR4.0/?page=InfragisticsWPF4.DataPresenter.v14.1~Infragistics.Windows.DataPresenter.Events.SelectedItemsChangedEventArgs_members.html

Now, the unfortunate thing is that your only real option would have been to use a TriggerParameterPath that looked something like this: "Source.SelectedItems", where "Source" would have been the XamDataGrid.  The unforunate part about this is that you cannot use Source or OriginalSource as the TriggerParameterPath.  I don't know why this is the case, but you can see this behavior if you try it with a Microsoft control like a TextBox.

What I recommend doing is creating a Blend behavior instead.  You can create the behavior and attach it to the XamDataGrid, then inside the behavior you can handle the SelectedItemsChanged event and manually invoke the command, passing it whatever arguments you want.

Let me know if you have any questions on this.

Sorry guys this should be in xamDataGrid - can someone move it for me?

Thanks