Is that ADE for android you downloaded? if so you dont really need that
I am on a Mac so the screens might be slightly different to the PC
Install ADE on your PC
the first part of this explains installing/registering
Adobe - Digital Editions : Help
find your book to download, this gives you a download link (.ascm file) which when you open this link in ADE will download the book and register the book to your adobe id and place the book in ADE
Then install the Android App (I currently use Montana and the option button is top right and then a section titled Adobe ID, I think Aldiko is similar but I havent looked for a while)
Copy the book over to the device (if you view properties of the book in ADE on your PC it will give the path for them) and import it into the reader
It is a lot simpler than it sounds, once you have the initial setup done it is very easy from then on
I have to go out for a bit now, but if you let me know the reader software you are using on the tablet I can check it out and see where the options are
Or just go with the reader for the store you get books from (I do use ADE like above for some things but mostly)
Kobo app for books from Kobo
Kindle for books bought from Amazon
Google Play Books for books bought from Google