When you start the EV3 with a leJOS SD card in the card reader, the Start-up Menu Java program runs automatically:
The first screen you see is the Run Default screen:
There are a set of top-level menu items that you navigate between using the Left and Right keys. To select a menu item press Enter. To go back to the top-level menu screen, press the Escape key. When a top-level screen is displayed, Escape will prompt you to shut down the EV3.
As well as showing a specific menu item (such as Run Default), the menu also displays your EV£ name (default "EV3"), the battery level and the IP addresses of the device. The top IP address is used for USB and Bluetooth connections, and the bottom one (if present), for Wifi connections.
Pressing ENTER on the Run Default screen will run your default program if you have one. See below for how to select a default program. If there is no default program a message saying "No default set" will be displayed for a few seconds.