Compiling Source packages in Mac OS 10.5.8

Dear R-SIG-Macers,

I am a very newbie for compilation of packages.  I usually rely on  
binaries in CRAN.  I need to compile a package under Mac OS 10.5.8.   
My questions are: a) Where do I put the .tar.gz file to compile and b)  
What are the exact command line instructions from the R.app GUI? I  
have RTM and I am lost as to what to do.

Particulars of my system:

Mac PowerPC with 1.8 GHz Processor and 1.5 GB Memory
XCode 3.1.4
GNU Fortran 4.2.3
Tcl/TK latest.

Output from sessionInfo()

R version 2.13.1 (2011-07-08)
Platform: powerpc-apple-darwin9.8.0/ppc (32-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

Thanks in advance,
Oscar A. Moreno
oscar.moreno at att.net