Do you see the links below the textbox? There are three link there, Tags, Files and Text. Click on Files and you can select multiple files to upload! You can only upload one file if you are using IE6.

Sometimes, you just want some of your friends to see what you have posted. You can change the share mode by clicking the button next to the Submit button. When changed to Friend Sharing mode, you can specify friends to receive your post. Remember that you need to follow your friend and your friend needs to follow back.