Use:
driver.findElement(By.className("AddContentBTN")).click();
In case you don't know, documentation for Selenium's "By" class can be found here.
Because I stumbled upon this while looking for my own solution, this will work as well. It is for C#
driver.FindElement(By.ClassName(classNameOfOuterDiv)).FindElement(By.ClassName(nameOfClickableDiv));