How to select the model selector for TI IBIS model of DRA725?
The Model Selector lets you select any of the listed buffer models in the list you show. So you can select buffer models Model_752 up to Model_759. 

The vendor has not done a good job explaining what the characteristics of each buffer type are to make the selection easy.

First question you need to ask: what IO interface is the buffer going to be used for? For example is this a DDR3 interface, is it a GPIO, etc. Once you determine that it will be easier to select the model you want to use.

By looking at the model descriptions you can see that half of them are for 3.3V IO voltage and the other half are for 1.8V IO voltage. So again depending on what type of interface you are using you can select the voltage you require. 

Finally you can see there is also definitions of Fast and Slow and 5 and 10 percent. The Fast and Slow are probably slew rate related if I had to guess. 

Sometimes when you download the IBIS model file there is a readme file or documentation that may provide more insight. Also you can search in the IBIS model file for the model name (for example Model_752) and see if there is more information listed in comments for that model.