MUPAD engine replaced Maple engine starting from MATLAB version R2007b+ and hence option was provided to switch to Maple engine if one has compatible version of a Maple software installed.
Entering the following command at the MATLAB command line would bring up a GUI for making your choice:
However, this option was made available in MATLAB R2007b+, R2008a+, R2008b, R2009a and R2009b. Starting from MATLAB version R2010a, the option to switch to Maple engine was removed and MUPAD was made the default and the only supported engine.
Since you mentioned that you were able to switch to Maple engine successfully I assume that you are using one of these MATLAB version - R2007b+, R2008a+, R2008b, R2009a and R2009b. If so, you can switch to the default MUPAD by using command 'sysengine' as shown above.