This is the screen after starting MMPlayer.

Tap "Playlist" and you should see this screen.

Tap "Add" and you will see the contents of your SD card. Note that it remembers the last folder you've been using, so you may have to navigate. In the screen above you see the root of my SD Card, showing four folders and a number of MP3 files.

Earlier I had copied the Seinfeld episodes using an SD card reader from my PC into a folder called Seinfeld. After tapping "Seinfeld" you can now see the Seinfeld movies (don't worry about the actual content of the screenshot, I had two versions of the first episodes with different encoding settings for testing purposes). Note that the little ".." allows you to go up one folder in the tree (in this example: back to the root of the SD card).

Select a file by tapping it, or select all files by tapping "All"

Tap "OK" to return to the Playlist.

Tap "Main" and then the play button.

Tap the screen once (sometimes twice...) and you go in Full Screen mode. Tap it again to go back to the buttons mode.