.mobi file location
Try to download/put the .mobi files on SD card in the following folder (not in Kindle folder):
Then open the Kindle application, and you will find it there (at least, I did).
The Kindle aplication will convert it automatically (in the background) from .mobi to .mbp and next time you go into that folder you will find 2 files for your book, one .mobi and one .mbp
I have an old GalaxyTab tablet with Android 2.3 (I think) and the Kindle application downloaded from GooglePlay, and it worked for me.
I have a question myself: the colors in the .mobi ebook opened with the Kindle aplication are wrong: blue is red, red is blue (for letters. I programed the colors in the .mobi file with Sigil/html tags). Other colors in pictures/images are bluish. However, green and fuschia letters are coming out OK. Any sugestions how to make it work properly?