If you would provide a little more information, someone here should be able to help you.
Such as:
What software are you trying to install?
'install dir' is meaningless on a Mac - Do you see that direction when you run the installer, or when viewing a read-me file that is provided with the software?
However, it is 13.00+ UTC (1+ PM UTC) and the 'Native Instruments' web site is currently offline. The estimate time for server return is 17.00 UTC (5 PM UTC).
Chill a little, barhar
The name of of the software is, in fact, Pro-53 - and is a software synth. 1.2.1 is simply the version number.
All this is not really in question, but just a detail - the install location - in the install instructions.
This can probably be interpreted as Applications folder, or just use the normal location that is used for installing other stuff.... I think you could install the software in whatever location is convenient.