Having followed the post from Kartie, I am having the same problem but cannot see a resolution in the posts. I have successfully created a custom material and saved this to the Public Materials for wall/concrete which shows that I can “Drag into PlusSpec” I get a white hand, but cannot drag the image into the “Drop Your Dragged Material Here” box in PlusSpec.
The main problem with this issue is that you can only drag and drop when using the Google Chrome Web Browser. Internet Explorer does not allow you to drag and drop.
If you are using Internet Explorer, to get your material that you have created you must go back into SketchUp. Once inside of SketchUp go to the PlusSpec Toolbar and you will see ‘SYNC’. Click ‘SYNC’ and it will download your created materials.