After importing/pasting, you'll get a popup about "table data was imported and can be adjusted". Click on that. Assuming no tabs are in your data, click on the tab to select it as a delimiter (it will highlight it green) and click on the comma to unhighlight it. Or type a custom delimiter and unhighlight the comma.
If Numbers doesn't see your import/paste as tabular data, which can happen if there are more "columns" in some rows than in others in the data, it will paste each row into a single column without having to do any of the above steps. This does not seem to be the case for you but, if you see that happen, this is why.