Ive seen it posted here a few times now so like the newsfeed fix i decided to share a workaround for those having trouble sending private messages via a mobile device (i had problems when using my blackberry)

The problem is the send button does not get displayed, i fixed this by searching my themes css file, its usually called base.css and will be located at /oxwall/ow_themes/your_theme/base.css

search the file for the following .ow_submit_auto_click

it should be displayed as .ow_submit_auto_click{display:none}, remove the display:none and it should fix it.

enjoy