I think there might also be an issue with what software development is made out to be - well defined tasks with outcomes and deadlines, supported by a manager and easily achievable in a the desired timeframe - and the realities of working in startup and early stage companies.
Self direction is the key here. All you really need is the outcome desired and the 'why?' - these should enable you to start and hopefully feel you are making an impact. Take it as an unbelievable opportunity, you're basically being given a project to go wild with, manage yourself and make your own decisions. Things might fail, it might take longer than they thought - but as many others have mentioned communication is the key!!! Write out YOUR plan, tell them what you're gonna do, give them an idea of time of the first few parts - and be realistic and transparent. This is pretty much all they can ask of you. Have fun and learn :)