Most frequently, this question comes up in the context of enabling the Active Directory plug-in.  However, any of the Directory Service plug-ins can be enabled in a similar fashion.  Here's how:

defaults write /Library/Preferences/DirectoryService/DirectoryService "[plug-in name]" "Active"

Substitute the plug-in name above to enable the plug-in in Directory Services.  Once finished, kill the Directory Service process:

killall DirectoryService

Wait 5 seconds for the Directory Service to restart.  After this it will be possible to create Search and Contact preferences from the command line.