a) Put the Archive vcfconvert-0.8.6.tar.gz into the "htdocs" folder of your Xampp Installation (i.e. /Applications/Xampp/htdocs or similar, i dont have OSX here).
b) Unpack the Archive. It should result in a new subfolder, i.e. /Applications/Xampp/htdocs/vcfconvert-0.8.6 (or similar, see above).
c) Start Apache
d) Open a browser (Safari) and enter http://127.0.0.1/vcfconvert-0.8.6/index.php
(or simply click on the link)
For your convenience, you may rename the folder from vcfconvert-0.8.6 into vcfconvert (or any other name), then you may also enter only http://127.0.0.1/vcfconvert/index.php
You also my try to enter "localhost" instead of 127.0.0.1 (this *should* work as well, but some computers are not configured sufficiently): http://localhost/vcfconvert-0.8.6/index.php