you cant use 3oc for that.
The preset meant for playing* samples as loops is called audiofileprocesser (AFP). Insert an instance of AFP in your project and use the file-vrose-icon on the preset to find any ogg or wave file you have on your pc.
Obs! Remember not to move og delete these files! LMMS looks for them every time you load your project
so.. what was the * for..
well 3oc does have a way to use wave-files. you can modulate the output-signal with a wave-shape. Here you choose the last option in the wave-shape selections, and browse to the file you wans as output-modulator. This is a serious creative tool, and you can make interesting modulations this way.