If another Open Diary member sends you a Friend request, you will receive a notification in the upper right of your screen. If you click on that notification, you will be taken to the Friend Request screen.
You can also get to this screen at any time by going to the main menu of the site and then clicking on Profile. This will take you to your Profile screen, where you can click on Friend Requests in the menu:
If you have any pending Friend requests, they will show on this screen. To accept or reject a request, click the appropriate link on the right of the screen.
If you accept a request, you will be Friends with that person on Open Diary until either you or the other person cancel the friendship.