I'm having trouble with your T-shirt designer. I can upload the .svg file I'd like to use no problem but it then doesn't appear in the "Your assets" tab. The cursor changes from a pointer to a hand, as if there's a grabbable object there, but there's no image! Can you help?
You might find it works if you change to advanced mode and:-
1. Click to add the object
2. Select object
4. Select top layer
5. Delete this obect
This will reveal your design if the SVG was create in Illustrator (as this software outputs the artboard over the graphics).
If this doesn't help then check your SVG only contains vector objects. Embedded images , for example, are not supported.