you need to run the create table for both the ServerList and also the dba_server_database_sizetrend. After you do that you need to insert data for each column example in code above.
example of insert for table
Alias = Server1.com
Connection = 10.10.100.1,1433
IP = 10.10.100.1
then you cut and paste the powershell script into a powershell file. Make sure you have codeplex SQLServer module installed. After you do this you can run the script and it will cursor trhough the list of servers in ServerList and populate database sizes in MB into the table.