It is quite easy to create one-way PPTP communication between Mikrotik and UTM, but the problem is if you want to make it two-way (problem is not Mikrotik but UTM related)...check my post about this issue:
p.s I have never found the solution, so I quitted this concept...it would be very interesting to try with SSL tunnel between Mikrotik and UTM (I thought that was not possible).