One way is to find a Computer Algebra System (CAS) that can import mathml. Unfortunately, while a lot of software exports mathml, almost none of it reads it. Here are some related links for a few cas systems:
CasADi (not strictly a CAS, but can evaluate expressions):
https://sourceforge.net/apps/trac/casadi/ticket/149
sympy:
http://code.google.com/p/sympy/issues/detail?id=2971
matlab/mupad:
http://www.mathworks.nl/help/toolbox/mupad/generate/MathML.html
