Hi Gijs,
Z. aethiopica is very easy from seed. If you have the whole berry, pop the seed kernel out and just cover with your growing medium. If you can keept them around 22-24C they should start germinating in 4-5 weeks, but make take 8 weeks for all to germinate. They will grow on well much cooler after germination.

The link below is for the best article I know of on Arisaema, I just don't agree with ever putting them in the refrigerator. If you have a cold basement or unheated closet that stays around 8C during the winter, I think that cool enough and a lot safer than the refrigerator.

A. maculatum is best sown outside as soon as ripe (June here) and germination starts sporadically in late October and November, then some more usually germinate in early spring. I have never known anyone to get packaged seeds that have been dry to germinate. These are more common in the U.K., maybe someone there can offer more help.

Enjoy,
Keiko

Here is a link that might be useful: Arisaema from seed