Go to the Advanced tab, then use the Insert dropdown and choose tab. (Because, as you found out, tab is an action character in this dialog box.)When my grandson showed me how to do FIND and REPLACE (find tabs and replace with space) after the table had become text, he could at first not do it because hitting a tab key for FIND makes the cursor jump immediately to the next line for REPLACE.
He then found in my text converted from table an example of "tab", copied it and inserted that copy under FIND with a space under REPLACE. Then hit REPLACE ALL and it worked.
(It took me a little longer to answer this because I went to check Pages '09 and it said a file was missing and I needed to reinstall. I have no clue why. Thank goodness I still have the install DVD. But I don't have a SuperDrive, so I had to get the external SuperDrive from my wife's iMac. Another pause for a little dropping accident I won't detail, then a 10 minute wait to reinstall and all is well. )