So my question is; does ID support csv, or do I need to translate the table to .txt with Excel on a windows PC?
Yes, it does. CSV and TXT are the only 2 filetypes InDesign can use for data merge - unless you use XML but that's a different thing.
Having never used Numbers to create such a file, I am not sure what the exact answer would be. However, a CSV file is comma-delimited so you could open the file in textedit and check to see if there are comma's after your column data. This would tell you if it was working properly. Also, make sure you are exporting using Unicode UTF-8.
Thanks for your reply..
I'm not sure if I used UTF-8, so I'll check that when I'm back in office.
I don't know what Numbers exports by default, or what it can export for that matter, but when you Select Data Source in ID there's an "Import Options" button that will let you select the delimiter, encoding, and platform of your data source. So it could be tab-delimited Windows Unicode or comma-delimited Mac MacOSRoman, or et cetera.