I've managed to add the \frown symbol using @"frown" : [MTMathAtom atomWithType:kMTMathAtomRelation value:@"\u2312"], in MTMathAtomFactory.m not sure how to implement \stackrel though and make it float about a given symbol. Do I need to add it accents or something?

\stackrel is defined as follows:

\def\stackrel#1#2{\mathrel{\mathop{#2}\limits^{#1}}}

So you could make an operator with limits as the \frown and value as AD. Note however, you may not want it to be a relation, as the spacing between AD and = is messed up.

Also you can add frown without modifying the source code as follows:

[MTMathAtomFactory addLatexSymbol:@"frown"
                            value:[MTMathAtom atomWithType:kMTMathAtomOrdinary
                                                     value:@"\u2312"]];

[MTMathAtomFactory addLatexSymbol:@"frown" value:[MTMathAtom atomWithType:kMTMathAtomOrdinary value:@"\u2312"]];
This code doesn't work the way i want it
How can I achieve the mathematical symbols in the picture above?
I found that there was nothe Unicode \u2312 in the given font package
I'm not sure if it's the reason why the code doesn't work

Add @"arc" : @"\u0311",
in the NSDictionary* accents in MTMathAtomFactory.m
seems works fine