From the FMP 11 Instant Web Publishing Guide, page 12 (http://www.filemaker.com/support/product/docs/fmp/fm11_instant_web_publish_en.pdf):
Restricting access except to specified IP addresses
You can restrict the IP addresses that can access your databases on the web. For example, you can specify that only web users from the IP address 188.8.131.52 can access your databases.
To specify which IP addresses can access your databases:
1. Open the Instant Web Publishing dialog box (see “Sharing a database with Instant Web Publishing” on page 9).
2. For Advanced Options, click Specify.
3. Select Accessible only from these IP addresses and type the IP address that can access your files.
Separate multiple IP addresses with a comma (,).
4. Click OK, then OK to save the changes.
Note Type an asterisk (*) in the section of the IP address to grant access to web users with similar IP addresses (for example: 12.34.5.* or 12.*). You can’t type an asterisk in the first section, however.
When I click OK at Step 4, FMP unexpectedly quits. This is running on a new iMac, Snow Leopard.