Me too facing the same problem. By any chance, did you find the solution for the issue?
I am facing the similar problem. Did you get a solution for this?
Haven't seen this post earlier.
The same question has been asked a couple of times, for example on the PowerCenter forum.
Basically the problem is that when you click the ODBC button in the Designer it opens the 64-bit ODBC administrator (C:\Windows\System32), not the 32-bit one (C:\Windows\SysWOW64). Usually the ...\System32 directory is in the PATH environment variable before the ...\SysWOW64 directory, and when Windows tries to open the application odbcad32.exe, it stops searching at the first match (which in most installations will be ...\System32, meaning the 64-bit one).
In other words: you have to define your ODBC data sources manually in the ...\SysWOW64\odbcad32.exe, then you can use these ODBC data sources in the Designer. But in most installations you cannot create / manage them from the Designer due to the PATH problem mentioned above.