The ones who downloaded the recent ecommerce trunk (currently r73999) might have noticed that you cannot get it running due to the missing payment module. There seems to be no link anywhere to download this new (and unsupported) payment module. However payments module trunk is accessible per svn, and assuming that you know svn and have it installed you can get it like this:
svn co http://svn.silverstripe.com/open/modules/payment/trunk/ payment
This will checkout the latest payment module trunk into the subdirectory "payment". Copy that to SS, run http://domain.tld/dev/build/?flush=1 and now the ecommerce trunk loads.
That doesn't mean that all works though, i have no products showing up anymore :(