It took me a while to figure out the logic for how Syncfusion is assigning a dataset that has already been created. When you add a "Data Source" to a report you are working on, the SQL Dataset name does not populate when you search for it. While in the Dashboard Designer application downloaded on your machine, follow these steps:
Select the plus sign in the "Data Source" box.
New Connection window will appear. Rename to the Dataset you will identify with. Under Connection Type, select "Microsoft SQL Server". Under Server Name, select "DATABASE 1". Maintain Windows Authentication. Under Database, select "ReportServer". Then click "Connect".
Then click according to the images below:
Then when you go to your working dashboard, start designing using the data from the set you require.