When someone want to give you access and share a folder with you, you need to accept the invitation that you received.
- Go on the dropbox website.
- Click on Log in on the top right corner.
- Type your Dropbox email and password and click on Log in.
- Click on the Sharing tab.
- Click on the invitation that you received.
- Click on Accept.
The folder should immediately appears in your Dropbox folder in your PC. So simple trick!