Had the same issue with Oracle ODBC drivers not showing up in the ODBC Manager tool on Win 7. Took 3 days but finally figured out that I needed to put the location of the Oracle ODBC drivers on the path environment variable. Then they showed up in ODBC Manager.
I'm having the same problem as well. I have the 32 bit drivers downloaded, I run the install, no error. I open either the 64 or 32 bit and no Oracle drivers are there. I previously had the full 10.1 client and I uninstalled that. It left reminants everywhere! So I manually removed the ODBC bits using regedit. They are gone. Now I install Instant client. SQL developer works. But no drivers for oracle are in the drivers.
Been working on this for 2 hours now!
I may have solved the problem. I ran the command prompt as administrator, then ran odbc_install.exe from the command prompt. It seemed to work. At least for the 32 bit. I can now see it in the 32 bit ODBC admin tool.
Nice of Oracle to let us in on that little secret. Maybe adding that to the readme would be useful??