I routinely use Excel for TDT and other formats. I've never had a problem getting Windows to open .tdt files in Excel by default; in Windows 7, it's right-click | Open with | Choose default program... | Browse | C:\Program Files\Microsoft Office\Office##\Excel.exe | Open.
However, I recently installed Excel 2013, and the above technique no longer works. Even after navigating to Excel and hitting Open, Excel does not appear in the list of programs.
So I guess I need to add the association in the registry somewhere. I see the list of extensions in HKCR, but the entry for CSV implies that Excel has special handling for that extension; anyway, copying the keys and value from .csv to .tdt has no impact. I also see HKCU\Software\Classes.tdt, with a "Content Type" of "application/vnd.ms-excel", but that doesn't seem to be doing anything.
Finally, there's HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts.tdt\OpenWithList. There's a value for EXCEL.EXE and several others, but only two of these (7-Zip and Notepad++) appear in the GUI. I can choose either of these two apps. However, it doesn't seem to have changed the content of either registry key, so there's no obvious entry to change.
Googling around, I see plenty of discussions about using the GUI, but the registry doesn't come up as often, and no two seem to be the same.