I will assume you already have a blank MoSync project setup that you can work on.
Scroll to the bottom of this tutorial to download an example project.
1) Download all the QR Decoder JS source files from here: https://github.com/LazarSoft/jsqrcode/tree/master/src
2) As per the instructions in the readme, include all of them in your index file (Android is case sensitive so make sure you check all the file paths carefully!
3) Download jQuery from here: http://jquery.com/download/ and include it into the head of your index.html file
5) Now add the button code below to call out getImage function:
If you are facing any difficulties, you can refer to the download at the above link to get all of the code in a package. Remember though, that the download contains files like the mosync debug script, which might now be outdated.