Hi Vallabh, you'll may well need to run the 32 bit version of Phoenix to import some of those files, take a look at the relevant section, p15, in the Phoenix 1.4 Getting Started Guide.pdf and perhaps try one of the examples from;
C:\Program Files (x86)\Pharsight\Phoenix\application\Examples\Legacy WinNonlin
You will be wanting to use the "WNL 5 ascii format" option under the "WNL5 classic modeling tab", if you have a problem simply paste the project here with reference to the one you're trying to replicate and we can advise further.
Bear in mind this format is essentially obsolete but if you want to create the same model in PML it shouldn't be too hard, there are already several examples on the forum. Normally the code is much simpler than the original.
Simon.