To access the IBIS-AMI model after it's been imported, you'll need to add a "Differential Buffer Element", which is on the toolstrip shown below.
After it's been placed on the schematic sheet, you right-click on the element and click "Select IBIS File & Model". From there, you'll be able to select and use the IBIS-AMI model you imported.
A good example to look at that covers several topics is the Analyze Backplane with Line Cards example.
