People often ask how they can check in on the Foursquare website. The short answer is “you can’t.”
Now for the long answer: Foursquare is a mobile game, designed to get you out from behind your computer screen and exploring the city around you. The mobile apps are designed to let you check in, while the website serves as a back end to edit venues, find friends, etc.
That said, there are perfectly good reasons why you might want to check in from your computer. You might take your laptop to the park on a sunny day to get some work done, for instance. While you’ve already got the laptop fired up, it’s easier to check in there than to pull out your phone just to check in.
For those times, you can use the Foursquare mobile website. It’s designed to be used on phones that don’t have apps available, but works just fine in any web browser. It will allow you to log in, see what your friends are up to and search for venues and check in.