To sign up for WHEN-sponsored field trips, click on "WHEN Events" at the top of the home page. (If you cannot see the "WHEN Events" tab, it is probably because you have not become a paid member. You must become a paid member to register for WHEN-sponsored events. Paid membership fees are what pay the hosting fee for the website among other things.)
After you have clicked on "WHEN Events", click on the little + next to the words "Field Trips" and you will see the field trip list. Locate the field trip you are interested in. (If you can't see the field trip you are looking for, go to the bottom of the page where it says "Display #" and choose "All" on the drop down menu.) Click on the blue "Read More" button next to the field trip you would like to sign up for. At the bottom of the page, you will be able to begin your registration.
Member-sponsored field trips do not always appear on the WHEN Events list, but are posted on the "Field Trips, Outings, and Gatherings" forum. In the forum posting, the sponsor specifies the registration information for the field trip.