When Blackbaud NetCommunity and The Raiser's Edge are both fully hosted by Blackbaud:
Set the plug-in URL in plug-in options to http://<siteid>.plugin.bbcust.com/pluginservice/masterservice.asmx

When Blackbaud only hosts Blackbaud NetCommunity:
Set the plug-in URL in plug-in options to http://<siteid>.bbnc.bbcust.com/pluginservice/masterservice.asmx

where <siteid> is the customer's Blackbaud SiteID number.