I agree that attaching a file can be a little tricky with this Forums software. Please try to follow the steps:
1) Click on “more reply options” (below the editor)
2) Click “Select file”. A Windows explorer dialog will open. Select teh file.
3) Now “Attach this file” .It will be uploaded.
4) Finally “Insert to post”. It will be inserted at cursor position, but you can move the placeholder using copy/paste.