Due to the way that Google Sites handles <A HREF...> tags when entered via the Edit toolbar's "Link" option, it is necessary to add links to FTP sites manually.  To do this, follow the steps below.

1. Navigate to the page on which you want the link.
2. Click the "Edit Page" button.
3. Enter the text you wish to use for your link.
4. Click the "HTML" button on the Edit toolbar.
5. Locate the text entered in step 3 above.
6. Surround the text with the following code:  <a href="ftp://ftp.example.com">.....</a>
7. Click the "Update" button in the bottom-right corner of the HTML edit screen.
8. Click the "Save" button on the Edit toolbar.

If you want the FTP site to open in a different window, simply add "TARGET=_BLANK" inside the <A HREF...> tag. 

Note:  Be sure to replace ftp://ftp.example.com with your desired FTP destination address.

Example:

CODE:     <A HREF="ftp://ftp.example.com/">FTP Example</A>

RESULT:  FTP Example

Have a question, but don't know where to start?  Check out the Google Sites FAQ!