Hi
The first step is the 64bit compiler of fpc. If not using fink, then you need to download the compiler AND its sources. The following command creates the 64 bit compiler:
make clean all OPT="-ap" CPU_TARGET=x86_64 OS_TARGET=darwin
after a successful build
"sudo make install" should install it.
As you may know the carbon based lcl will not work, since it is 32 bit only. These are your choices using Lazarus:
1) cocoa-lcl
It may not have all components, but is closest to "native" look and feel.
2) qt-lcl
More components than cocoa-lcl, still fairly close to "native" look and feel. Needs the installation of qt4.
3) gtk2-lcl
Probably the largest number of components, but could be further from a "native" look. Needs the installation of all the gtk libs.
I prepared package description for fink, the Debian like package manager. Installing fink is some work, but once that is done, choosing and switching between the three options above is easy. Fink takes care of all dependencies, i.e. fpc and lazarus as required, qt and/of gtk2 libraries and so on.
If you do not want to use Lazarus and the lcl, then you can go really "native" using Xcode for the design of the UI and plugin your pascal code. Phil is the expert on this.
I hope this gives you a rough idea and directions about your options.
MiSchi