First you need a source image:
Now you should see your image in the right view and a thumbnail of it on the left. The thumbnail shows a small version of the output image, giving you an impression of its contents and format.
If you only want to enlarge a part of your input, you can define a cropping rectangle in the right 'Cropping' view by dragging the mouse with left button down. You can resize or move the cropping rectangle by grabbing its frame or interior. Watch the contents of the 'thumbnail preview', they will change accordingly.
In the combo box under the picture you can select one of several fixed formats for your crop, or use 'free' to allow arbitrary cropping.
Under the header 'Output Dimensions' you can choose between several methods to define the size of your output. For example you might select 'Specify zoom factor' and move the zoom slider. The output size shown under the thumbnail will change accordingly.
When you are satisfied with format and size of your output, press 'Enlarge & Save'. After the progress bar reached 100% the result is automatically saved to the file and location named under the header 'Write Result to:'.
Most interface elements have tool tips (they pop up when you don't move the mouse some time).
By the way: For big zoom rates you might get better results when doing the enlargement in multiple steps:
For example for a 16-times Enlargement you could first enlarge with 400% into a temporary file (best not to use JPEG for this to avoid artifacts) which you then load and enlarge by a further 400% to the desired size.