For private messages, hover your mouse over your username in the top right hand corner. Click on 'User Control Panel' then on 'Private Messages'.
Alternatively, to message Andrew, you can click on the little button that says 'PM', under the picture of a lion next to any one of his posts. You can do this with any person here.
Tapatalk did an update recently and a few things have changed which have made it harder to navigate so I tend to use the website on my laptop which I find much easier.