Oh, now everything makes sense, ESXi free hosts are not visible in advanced scalability mode. Could you tell me how many hosts are you currently monitoring? Unless you're monitoring a very large virtual infrastructure, I would recommend you using typical monitoring mode.
Because typical mode is intended for monitoring virtual infrastructure in the real time, storing data within a limited time range and/or collecting data for not very large VI environments.
And if you choose advanced mode, it is intended for analyzing data within great VI deployments using a number of vCenters. The advanced mode can be used if it is necessary to trace the general dynamics of collected data, instead of the real-time data monitoring.
But if you've just added vCenter connection to Monitor application, you should be able to see all the performance data for your Virtual Objects in both modes.