I was at Immigration on Friday to apply for a month's extension and although the form stated a TM6, it wasn't required. ( temporarily suspended last year but back again from May 1st this year, see article in Bkk Post.)
LaVera , Rest assured, yu are fine, as everybody here says. With a US passport , yu get 60 day visa-exempt in Thailand! So, there's no need to apply for a visa! Chill , enjoy yr trip! (: