http://java.sun.com/javase/downloads/index.jsp
Download the JRE (Java Runtime Environment) which corresponds to your operating system (Windows, Linux ...), and install it.
http://xlogo.tuxfamily.org/common/xlogo.jar
Otherwise, you can go on the XLOGO website, at the address http://xlogo.tuxfamily.org, choose english language and then click on the downloads menu.