several ways of achieving this.
$currentuser is available and can be shown on a page by using a data source microflow for example.
use the online mendix training for all possibilities
As Stephan says there are several ways, for a possibility would be:
Added the screenshot so you can see in which entity it is located
If you want to display the FullName, you can add a Inheritance split and Cast activity on the $currentUser variable, to cast it to an Account object, which has the FullName attribute.