InstantCake doesn't provide any networking functionality, whatsoever, so there shouldn't be anything in your rc.sysinit.author file, to that effect.

Instructions for changing from DHCP to STATIC are documented in our knowledgebase here:

Changing DHCP to STATIC with PTVnet

And i'm almost certain this information is consistent with the sticky thread posted in these forums.  In the default PTVnet rc.sysinit.author configuration file for 6.3, you will need to add the IP information listed in the article.

For answers to commonly asked questions, please refer to the
InstantCake Instructions and Release Notes,
Official InstantCake Discussion Thread, and the
Official PTVnet Discussion Thread