What program(s) can I use to open an .xlsx spreadsheet for editing on my Mac I can double-click the file and a browser preview opens that looks like excel.... but it is basically just a browser preview feature and it is read-only.

The file came from a Windows machine.
If you have opened it in Excel but you are unable to select or change it, the sheet may be protected.   If the sheet has been protected using the Tools > Protection > Protect Sheet option then a lock symbol will appear next to the sheet name in Excel 2011.

Sheet protection can be removed by using the Tools > Protection > Unprotect Sheet command.