Looks like we have some runaway confusion building here, so I will attempt to clarify the clarification. The problem is that posts in this thread are using the letters "emb" in two different contexts:
The original post was about the free designs included with the RE18 hoop, and that they could not be opened in the MC15000. Some people could, some could not. Cat posted a list of the files contained on that USB stick, which showed a large number of designs in the EMB folder. A follow-up post was made stating that files in emb format cannot be opened in the machine. This is a true statement, but it is not related to the original post. Unfortunately those 3 letters have different meanings depending on where they appear.
EMB as a folder name on a USB stick is a convention on the Janome machines starting with the 11000. It is a top-level folder that contains at least one folder inside it called EmbF. This folder will be created on any USB drive plugged into the machine, if it is not already there. By default it is the one first opened on the 15000 when you choose to open a design on USB, but on the 15000 you can use the keys on the screen to go to other folders.
.emb as the extension on a file name is the default for embroidery designs created in Wilcom digitizers. Such files cannot be opened by the machine, but must first be exported to JEF, JEF+, or JPX in order for the machine to open them. Again, this applies only to the names of design files, NOT folder names like the EMB on a USB drive.
Going back to the original post, I checked to see if we had the elusive designs from our purchase of RE18. Apparently we do not, or if we did we did not save them. As I recall they were not "world class", and the main appeal was that they were free.