Depending on your browser, there may be no sane way to type tab characters into a textarea.
Alt-keypad0-keypad0-keypad9 might work, or you'll have to copy and paste an existing tab.
This matter is covered in detail in TipForTypingTab, which should be read before voting below.
I suggest a vote (add Y to agree, N to disagree): [YNN]
How about moving "I can't type tabs" up, close to the Save button? -- BenTremblay
See also: TabMunging