It's not straightforward but this works just fine.

 1. click on your name up in the upper
    right hand corner
 2. click on "System Preferences..."
 3. click on "Main Menu" in the
    Look-and-Feel section of the
    "Control Center" that pops up
 4. select the section where you want to
    add the software-center launcher - I
    chose "system tools"
 5. click the "new item" button
 6. enter "software-center" as both the
    name and the command 
 7. click "ok"
 8. click "close"

Now when you select "Activities" in the upper left hand corner, then the "more" button just to the right of the word "Applications", you'll see your new software-center launcher listed in the system tools menu.  

I tried it - it worked perfectly.