LED modeling with Shockley diode equation
There's a few things that you can do:


Find a SPICE model for your particular LED. SPICE uses the Shockley diode model, so you can just pull Is and N from the model card.
Find a datasheet for the LED in question (with a graph indicating a voltage-current relationship) and model only Is and N, which is actually not very difficult. See my answer on "How to model a LED" for most of what you need to get started. Only solving for Is and N will work well until you start trying to use the equation for peak currents (high current, low duty cycle).
Take the forward threshold voltage of the diode, and solve the diode equation for Is. Guess about 1.8-2.0 for N and just be satisfied with the result. It won't track reality very well, but it will behave like a diode.
Take the values for this red LED and hope that they mostly work for your LED. 

IS=1a RS=3.3 N=1.8


Keep in mind that different color LEDs have different forward voltage drops, and that even diodes with the same color can have different voltages. Also, the diode equation neglects series resistance, which can have a significant effect at high currents. It is up to you to understand your accuracy requirements, and to know the limitations of your calculations.