I'm using Qucs for a simulation, but I don't understand how can I create a new component (E.g. a new LED from a data sheet). How can create it and use it easily in my project?
If you look at the entries in the existing 'LEDs' library (LEDS.lib in subfolder Qucs\share\qucs\library of my Windows Qucs installation) you will see how these are defined.
You do need to understand how the models work though. In this case, the LED is based on the model parameters for the PN Junction Diode, which are documented in the appropriate section of the technical documentation: http://qucs.sourceforge.net/tech/node68.html
The dc characteristics of the diode are determined by the parameters Is (saturation current) and N (emission coefficient). An ohmic resistance, Rs, is included. Charge storage effects are modeled by a transit time, Tt, and a nonlinear depletion layer capacitance which is determined by the parameters Cj0 (zero-bias junction capacitance), Vj (junction potential), and M (grading coefficient). The temperature dependence of the saturation current is defined by the parameters Eg, the energy bandgap and Xti, the saturation current temperature exponent. The nominal temperature at which these parameters were measured is Tnom. Reverse breakdown is modeled by an exponential increase in the reverse diode current and is determined by the parameters Bv (reverse breakdown voltage) and Ibv (current at reverse breakdown voltage).