I do not have experience with this plant, but in general I have found that seeds from exotic Solanum species are easy to propagate in warm soil -- provided they are from a ripe or over-ripe fruit.
I assume you know the fruit is poisonous?
Here's species information from GRIN and Wikipedia:
Solanum mammosum information from NPGS/GRIN
Solanum mammosum - Wikipedia, the free encyclopedia