How do I open an ERwin Naming Standards file (.nsm) in Excel?
First, you need to load the .nsm file into the model. This is done through Tools > Names > Model Naming Options... > Use File: (select the appropriate file).

Well, if you've got version R9 or higher, you're in luck. As per this post, the naming standards are an object in the model explorer just like every other entity, so you can simply find it in the model explorer, right click on it, select properties, and on the glossary tab click the save button, and save as .csv.

For pre-R9 versions, you need to go to Tools > Names > Edit Naming Standards... The Naming Standards Editor window should pop up. There's a few tabs near the bottom, click the one that says 'glossary'. You'll now have an option to export the file as .csv.

Took me way too long to figure this out myself, so hopefully this could help the next guy who has the same issue. ;)