Hii! I'm not a crazy big Mecabricks expert, but I can try and help.

1. Go to/open one of the models you want to have in the large model, in workshop.
2. In the upper left corner, click on File.
3. Several options will show up; you need to click on Import.
4. Once you do, it should be on the Mecabricks option, but there are two more options, like LEGO Digital Designer. If one of the other options are selected, reselect Mecabricks.
5. Click Import again.
6. It will then show all of your models. Pick whichever model you wish to import, and click on it. It should show up with the other model(s) on the graph.
7. Repeat!
   Obviously this won't work if you are attempting to import someone else's model; in that case, tell me and I'll rewrite these instructions to fit.
   P.S. In the future, if you ever have more questions, simply scroll through the "Help and Tips" and "General Discussion" forums, or even the "Community" forums, and there is a high possibility that you will find someone who has already asked that question, or even just posted the answer for the convenience of users. 😃
   Feel free to Private Message me or my brother, @SlimBrick1 (who knows a LOT more than me), and quiz us some more on the workin's.
   Good luck,
   Tall.

Thanks @TallGirl2006 ! Yes, I am trying to import models from others. Also, I love your HoneyLemon! (I presume that is who your avatar is!)

To import models from others... Follow the same steps as above, all the way to step 5.
It should, again, show all of your models. Above your models, it shows your username, highlighted in green. Next to your username, there, are three other options. These are everyone else's models!
From there, it's pretty self-explanatory. You choose either moc (which is the majority), set (which is actual LEGO sets), or minifigure. There you go!

Yeah, that's Honey Lemon! I chose her for the name Tall Girl, her fanfic name in the show. I actually didn't build the original brickhead; I only updated one (by clipping) that someone else built. His name is @Icebear. I highly recommend checking out his brickheadz; he's amazing at building them- part usage, accuracy, everything!
https://mecabricks.com/en/user/Icebear/models?p=3

Thanks a lot for the help!

Not a problem! Have fun with Mecabricks. 😄