Help to read books om Disgo 6000 tablet

Hi, I wondered if anyone out there can help me with a problem.

I am a complete newbie when it comes to tablets, I've never owned or used one until now. I've just bought a DIsgo 6000 as a Christmas present for someone and want them to be able to use it doubled up as an e-reader.

I've put a load of books onto an SD card and inserted it into the tablet however when I try to open any of the books, none of them will open. I've tried opening them by just double clicking on the book and they wont open, or I've tried going into DroidReader, which was already installed ont he tablet, only it doesn't find any books.

I've tried googling (which is what brought me here) and I see lots of people recommend Aldiko to read books with so have tried to download that but I can't seem to figure out how to get that app ontop the tablet (yep, I really am a newbie!!). I also tried downloading Aldiko via the laptop and saved it to the SD card but I still can't quite figure out what to do with it (or if that will even fix the problem).

It shouldn't be the book files themselves that is the issue as I have tried loads of them, all of which work on my own (non-tablet) e-reader.

I'm at my wits end as I really don't know what I'm doing and wondered if anyone out there could give me some pointers?