How to pick node number for datum planes?

If you are using planes with known names to make the cuts, then you can build a search query to select each plane in the mapkey.  Use the search tool to build a query to find a plane of a specific name as seen below. In this example the wildcard returns all named planes in the results. Use a specific name to return a single plane. You have the option to save a query to a file as well if you want to reuse it for more than one mapkey sequence.

Have you considered using a UDF to add the datum planes? It's a little more setup up front but could make your mapkeys simpler. We have a lot of mapkeys that call a UDF and then reference the UDF features after creation for other features. 

http://support.ptc.com/help/creo/creo_pma/usascii/index.html#page/part_modeling/part_modeling/part_five_sub/About_User_Defined_Features_part.html

https://www.youtube.com/watch?v=xI9T1fGHZGI

I don't quite follow what you're trying to do, but have you considered pre-selecting the datums before recording the mapkey? This would then become the pre-requisite for running the mapkey. User selects the two planes, then runs mapkey.