If you want to send a PM you need to have at least 15 posts, otherwise you will not see the option to start a new conversation. Make sure your posts are good quality and do not make short, spammy or low-quality posts just to boost your post count - these posts will be deleted and your account may be banned. When you reach 15 posts, you will see a yellow button appear at the top of your inbox with "Start New Conversation" - this allows you to send a private message to another member. You will also see the option to 'Start a conversation' in the pop up window when you click on another user's name. Do not send PMs promoting your site or services or asking for things - this is against the rules and may lead to your account being banned.