As far as I know you are not able to select an instance in a linked model.
To access the information from a linked model you can look at a post on building coder.
http://thebuildingcoder.typepad.com/blog/2009/02/list-linked-elements.html
you can try this to work with a linked document
Application app = commandData.Application;
DocumentSet docs = app.Documents;
foreach( Document doc in docs )
{
// You can check the name of doc.PathName to see if it is the linked doc you are looking for
// Then you can get any information you would like from the doc.
// I think maybe you would like to do a type filter and find all text and then process them for future actions?
// Unfortunately I don't think you can get coordinates of a mouse click but you could have the user select an object in the active document
// that is closest to the text, store the location of the selected object, and then search the linked document for the text object closest to th
// the location.
}