It is a known issue that Mac clients can not use the attachment (upload) button when posting in the forums module. Researching this appears to be a limitation on the Mac we believe it's a security restriction.
There is a simple workaround when using the large forum browser. Drag the file you wish to attach over the message area of the post, this will perform the same function as pressing the attachment button. Also, You can upload files to file sharing sites and link to them.
Drag the file you wish to attach to the message area of the post you are creating.
After you have dragged the file into the message area, it will appear in the attachment box.