Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
If you like to introduce yourself, to the community, go here:
http://lmms.io/forum/viewtopic.php?f=4&t=4480
You need to refresh the sidebar-view.
Press the circle-arrow:
-Here its in lower right, that may be different, but the button looks like it :)
After that refresh, you will be able to see newly Windows Explorer added resources.
That is true! It is a license issue, and you need to use one of the formats that lmms supportsAudioFileProcessor doesn't accept .mp3.
* wave
* ogg
* ds
* flac
You can use a program like Audacity to convert your MP3 to any of those, however! Do remember that MP3 is a lossy format, and you do loose some frequencies when you use MP3, so it is even not suitable for samples. Get you samples in better versions. Preferably wave or Ogg