The Equation Editor (EE) in Microsoft Office isn't compatible with MHF. I should say neither EE in Office is compatible. Not many people realize there are two. There's the "new" one (officially the "OMML" editor -- OMML = Office Math Markup Language) of course, but Microsoft Equation 3 is also there. This is the one you've seen in Office since Office v2.
Like I said though, neither of these is compatible with MHF. However, the upgrade to Microsoft Equation 3 -- MathType -- is compatible. You create the equation visually, like you would if you were adding it to a Word document, then after choosing the Math Help Forum "translator" (from the Cut & Copy Preferences dialog), you can paste it directly here. MathType includes all the correct codes -- not only LaTeX, but the opening & closing codes to distinguish the equation as being "math".
More information about this capability at the Design Science website.