Set up X11 so that you can cut and paste:
X11 → Preferences → Input → Emulate three button mouse
Now <option/alt> mouse button can be used to paste from the clipboard
Set up X11 so that focus follows mouse:
In any xterm, type
defaults write org.x.X11 wm_ffm -bool true
then restart X11 for this to take effect.
Install XCode so that you can compile:
XCode is on the DVD that comes with your Mac; you can also download the dmg from apple.
Install gfortran:
Choose the approprate dmg from http://gcc.gnu.org/wiki/GFortranBinaries#MacOS