Do you mean the 430 series flash? That is an older flash that I imagine has been out of production for sometime. Sigma may take a few days to answer questions and if your subject line has 340 in it and they are using an automated filter, it probably does not know what to do with the query. The flash may not work with anything other than a Canon digital camera. And it may not work with all of them either. These flashes are system flashes and don't work off a PC sync port.

I'll check some sources and see if anyone I've communicated with has a manual, but it probably is just matter of scrolling through some MODE options on the flash until it says M or Man or Manual or something like that on the display. You should be able to then set different fractional levels of output.