The Windows MAMPlayer application is freeware, offered without technical support.
That said, this page may help you solve (or learn that you cannot solve) the most common technical problems users encounter.
Remember, you're running Microsoft Windows, which means that the standard Microsoft advice applies: "Reinstall the application and restart Windows". In the case of the MAMPlayer, there is no installation process, so all you need to do before restarting Windows is to erase the mamcfg.txt file.
By default, MAMPlayer uses the same MIDI playback that's used by other programs (like WinAmp, RealPlayer, Windows Media Player, etc.). Try playing a MIDI file with one of those programs to confirm that your MIDI playback is working okay (if it's not, solve the problem for the other players before attempting to run MAMPlayer).
If other MIDI-playing programs work okay, it's possible that the MAMPlayer is set to use a different MIDI device than those other programs; select MIDI Out in the MIDI menu, and try selecting a different device.
Yes, we know. This is a limitation of the program. There's nothing you can do about it.
There isn't a simple way to do this, but if you're willing to go to some trouble, see the page How To Make Music Animation Machine Videos To Upload To YouTube.
This is probably because your computer isn't fast enough (and the program is inefficient). Stephen Owen wrote to say "I found that if the user opens up task manager and sets the app to a single processor affinity, most slowness is resolved." Beyond that, the only thing I know of that you can do (besides getting a faster computer) is change the view so that the range of time being shows (and therefore the number of notes) is smaller.
There are lots of things that can go wrong; here a some things that people have tried that have helped:
If you've tried everything and you're still having problems, there's probably nothing I can do. However, there's no harm in asking. Send a complete description of the problem (what your setup is, what you tried, what happened) along with a copy of the two files MAMPlayer creates (mamcfg.txt and mamlog.txt) to me via email. If I don't reply, it's because you didn't try everything, you didn't include a complete enough description, or I don't have any suggestions. If you are desperate and are willing to pay for tech support, my rate is $250/hour, with a $100 minimum; send me a $100 retainer via PayPal (use the email address on my contact page) with a description of the problem and your email address. If it's something I can help with, I'll accept the PayPal payment and contact you; otherwise I will refuse (or refund) the payment .