Hi Alvin,
I'm not exactly sure what you mean by "download DNA to WikiTree" and "download my DNA from my Family Tree" but I'll attempt to answer...
The first thing to clear up is that WikiTree does not store any raw DNA information. When you see a profile that indicates that the person has submitted DNA information it merely means that they have added an ID code of their raw DNA information to their profile that links back to their account at the testing company (FTDNA, Ancestry, etc) If you have completed a test you can add that information to your own profile; I forget the exact details bit I think you can click on the ADD menu from your own profile and then select the DNA option. You only need your test ID from the testing company.