Thanks for your update.
We suspect that FontListBox and FontComboBox controls are not added to your Toolbox. We request you to follow the below steps to add it to Toolbox
1)Right click on the Syncfusion WPF Controls in Toolbox as mentioned in below screenshot
2)Then search for the FontListBox control using filter in “Choose Toolbox items” window
3)Add it by clicking “OK” button and now it will be visible in Toolbox
Can you check with the above suggestion and let us know whether your problem is resolved?