Now that's easy. You have two options:

• install using MacPorts, or
• use the binary installer (which, in turn, is based on MacPorts).

Both options will give exactly the same results.

• Download and install MacPorts, as described here.
• In the terminal, type:

sudo port install swi-prolog

• You're done!

• Go to the SWI-Prolog download page and download the appropriate installer for your architecture (Intel, PPC) and Mac OS X version (Tiger, Leopard).
• Extract the mpkg file and follow the installer instructions.

Both methods install SWI-Prolog in /opt/local/:

• the binary is in /opt/local/bin/swipl (add /opt/local/bin to your PATH if you - or the MacPorts installer - haven't already done so)
• all the rest is to be found in /opt/local/lib/swipl-<version>

To test if it works:

• go to /opt/local/lib/swipl-<version>/doc/packages/examples/jpl/java/Test
• just to make sure: rm Test.class
• ./run.sh - this should compile Test.java (in the absence of Test.class) and run it.
• If all goes well, you should get:

bash-3.2# ./run.sh
Compiling Test

JPL demo: Test

test 0...% test.pl compiled 0.00 sec, 4,064 bytes
passed.
test 1...passed.
test 2...passed.
test 3...passed.
test 4...passed.
test 5...passed.
test 6...passed.
test 7...passed.
test 8...passed.
test 9...passed.
test 10...passed.
test 11...passed.
test 101...00004567182904567231901235678312041678321528904567312936890412738905683412794156842736189057340412506823420839420831756240819623751407856239104837562910482956804237195684739158764391584763596579576799passed.