Since the relationship to MyItem is usually 0:1, you probably will find it if you scroll the drop-down list further down (after the "Is Related To" entry) until you reach something like "My Item via IncidentAssociatedMyItem".
Probably I don't understand exactly what you want to do, but as your list shows, you have already selected the "Incident via IncidentAssociatedMyItems" relationship. Therefore, you should be able to base your search also on values in the related incident fields.
Maybe it woudl be helpful if you explain in more detail what you want to achieve.
We moved our mailing lists to "Contact Groups" to have it organized in HEAT.
One of the group is able to create Incident through the service catalog.
The generated Incident is then visible as a FRS_MyItem.
In order to make it visible to the whole contact group. I assumed I could add a pick list of Contact Group in Incident and FRS_MyItem (and have it populated properly).
And then control its visibility through a search.
Now I explain it I'm not even sure it's possible