To enable mod_rewrite manually, see: OS X server tips but if you are using VirtualHostX it changes your default config file. Add the following code to the Directives box:
Options FollowSymLinks
AllowOverride All AuthConfig
Order deny,allow
Deny from all
if you’re using the new version of VirtualHostX, make sure that you select Directory from the dropdown, then use:
Options FollowSymLinks
AllowOverride All AuthConfig