Make sure the game is in a directory with only English letters (no Japanese characters, no English letters with accents, etc).
When you select the folder, make sure the installer isn't appending an extra folder name to the end of the directory; if it is, just delete that extra folder name.
Edit: I successfully unpatched and repatched my copy of DDC.
I did the following: open the installer, click Yes, click ??..., select my DDC directory, click (OK).
The installer then patched the following 4 files: custom.exe, readme.txt, th14.dat, and th14.exe.
Open DDC and look for the version number at the lower-right corner of the main menu. What is that version number?
What files are currently inside your DDC directory?
What version of Windows are you using?