The switch itself is -I, not -Ipath. 'path' indicates a parameter for which you need to substitute something, in this case a directory path. It should be the root folder for the source modules you want to add to the search path. In this case, for artemisd, the source files are in the 'source' directory, so -Isource is what you pass to dmd. If the directory were named 'foo' instead, you would pass -Ifoo.
aha! that would've bitten me in the future, now I get it, thanks.
