To use the MuPAD OpenMath Package, just download one of

 OpenMathPackage-x.y.z.tbz      
 OpenMathPackage-x.y.z.tgz           
 OpenMathPackage-x.y.z.zip           
 OpenMathPackageNoSCSCP-x.y.z.tbz    
 OpenMathPackageNoSCSCP-x.y.z.tgz    
 OpenMathPackageNoSCSCP-x.y.z.zip    

and uncompress the archive. 

Move the resulting 'OpenMath' directory to any location appropriate, 
possibly into your MuPADs PACKAGEPATH. 

Start MuPAD and issue

>> package("OpenMath");

if that doesn't work out please consult the MuPAD help pages for
'package' and/or 'PACKAGEPATH'.

If the package was loaded successfully, you'll see a message like

Package 'OpenMath' x.y.z loading. To get further information try OpenMath::doc()

and you can get further documentation with 

>> OpenMath::doc()

Note that SCSCP support is disabled in the NoSCSCP packages.