It is advisable to read the tutorial in the sequence presented, even if you do not anticipate using every feature discussed. Each section presents concepts that are generally useful. File names, numbers of files, camera names, etc. will not correspond to yours.
See also:
Starting out:
· Prepare for processing: Create Profile
Create Calibration Masters (optional):
Stacking:
· Grayscale (monochrome camera)
· RGB Color (monochrome camera)
· Bayer Color (one shot color)

The opening screen contains only three possible actions (other than closing):
New
Job opens the Control Editor
Open
Job or View Image
Click
(Open Job or View Image) to open a File
Dialog (actual apearence may vary according to Windows version):

Use the File Type
drop-down box to select “all images”. Navigate to the desired folder
and select an image file. If more
than one file is selected then a separate Viewer will
be created for each file.

Take some time to become familiar with Viewer controls. Other than looking at the image, there is not much you can do with it except to save it in another format (e.g. save a JPEG from the scaled display). A viewed image is not automatically used for stacking or other processing, so you cannot start a stack by selecting files from that dialog.
Click
(New
Job) to open the Select Task form:

Stacker Unlimited needs some information to profile the image source, usually a Camera. To create a profile, press the Utilities button:

Press the Manage Profiles button and follow the instructions here.