You should create a GEDCOM file from your MyHeritage tree.
I have used the MyHeritage Family Tree Builder software package (this is the free software that lives on your own computer -- you should have saved a copy of your online data in this software) to generate GEDCOM files. Under the File menu, select Export GEDCOM.
The next screen has a fill-in blank labeled Save As. It never works for me (your mileage may vary), so I just hit Next. The Next screen will prompt you with a suggested name for a GED file; you can accept that name or create some other name. After that, keep hitting NEXT until you get to a screen that asks you what you want to export. You should export notes, sources, and citations, but not photos (because the WikiTree utility does not import them).
You can choose to export the entire tree or to include only selected people. In the past, I was able to select a branch of the tree (for example, all descendants of a particular person), but that option no longer seems to be available for me... It is generally best to start with a very small GEDCOM file so you can test the WikiTree GEDCOMpare software -- and WikiTree does not allow upload of more than 5000 people (which I would call a huge GEDCOM).