By mistake I have installed 64-bit linux(hey! it happens to every1!). How can I run 32-bit app under that linux?
Most applications (Well, all applications) that are in the repo will work with 64bit versions of Ubuntu. So things like Firefox, Gedit, Chromium, etc. The majority of applications that are only i386/x86_32bit only will still install and run without any additional changes needed. Even less will have issues with linked or shared system libraries. You can install the
ia32-libs package which will install most needed 32bit shared libraries needed for 32bit software to work.
I don't see how installing a 64bit OS on a CPU that supports it (most have done for years now) is a mistake, it makes perfect sense to make use of the benefits of 64-bit architecture! I must say, for at least the last few years, apart from 64bit flash issues, I've not had to worry about this at all.
Saying that, what are you trying to run? My system has a few 32-bit libraries and i386/i686 .deb's I've force installed (which seem to be architecture independent anyway), and you might notice that your's works the same.
Does the app you're attempting to run work on the 64bit system?