1. Check that you have entered the coordinates correctly.
2. If you have zoomed in too much, the image will be hidden so you have to zoom out before you can show the image. (It sounds like you already tried that though.)
Does the problem occur immediately when you first try to import the image.
The problem is that when you zoom in, ModelMuse tries to create a larger version of the original image and then displays the part of the image that is in the field of view. If the resized image gets to large, it causes a problem and ModelMuse responds by hiding the image.