First of all you would need an up-to-date revision of the Open IMS Core. So before you begin, upgrade to the latest revision in the SVN (IPv6 was introduced around r529, but you should always update to the latest revision that contains all the bug fixes).
Then you need to change your DNS and configuration files to make use of IPv6. For DNS, if you would like to have a FQDN (domain name) translated to an IPv6 address instead of an IPv4 one, use an AAAA entry instead of an A one:
pcscf4 1D IN A 127.0.0.1
pcscf6 1D IN AAAA ::1
Yes. In case your svn checkout should not work, you can download the appropriate project from our ftp site:
ftp://ftp.berlios.de/pub/openimscore/snapshots/
You really need a DNS server. For example the P-CSCF will do a NAPTR/SRV query to find the I-CSCF and only a true DNS server will be able to solve this. If you need help with that, please read this.