Not any diode but the Diode (PN-Junction Diode), should be right next to the Diode_Model (PN-Junction Diode Model) on the same palette. These work as a pair the Diode give the connectivity and some data to determine things like the size and temperature of the device and the Diode_Model sets the Spice type simulation model parameters. The first parameter of the Diode is "Model" and should specify the instance name for the Diode_Model symbol, for example DIODEM1, and that is how the two parts of the data are linked. Both parts are required.