You can’t easily freeze rows or columns in the “Excel sense” in Numbers–Apple’s version of a spreadsheet.
But what you can do is create header rows and header columns (the default grey ones) that ARE frozen.
>> open the document inspector
>> table inspector (3rd option for me, looks like a yellow table)
There are three “headers” and “footers” drop downs. At the bottom of the first two is an option to FREEZE that element. This only applies to header and footer rows/columns, but that should about solve the problem. It’s not as nice as excel’s approach (which in my opinion is a little ‘cleaner’), but it works.