You're going to have to ask informatica how to do that. The best advice I can give you is to install a 32 bit Oracle client, and use the 32 bit ODBC manager to create the connection. That will get things up and running at least.
Did you get any answer on this? Even i see same issue... I have Win 7 64 bit OS and 64 bit Oracle and 64 bit Informatica. Not able to create ODBC connections because informatica is pointing to 32 bit ODBC.