1. Sign in to dropbox.com
2. Hover your cursor over the file or folder you'd like to share and click share when it appears.
3. If a link hasn't been created, click Create a link. If a link was already created, click Copy link.
-- After that you can send that link to anyone.
In your messages area with client, on the left of the chat window is a paperclip, click that and it will give you two options: one Choose File or Share via Dropbox. If you choose Dropbox it will open your dropbox and ask you which file you want to send. I do this alot.