I have not tried to access ngspice from KiCAD, but providing a little more information may help you get useful responses.
Are you having difficulties getting ngspice to run when called from within KiCAD? Any model for a 555 timer must be produced as a SPICE "subcircuit". I recall some recent posts about passing information for SPICE subcircuits from KiCAD to ngspice. Search for threads with the keyword "subckt".
Is your problem more related to modeling the 555 timer, or simulation in general? Even though the 555 has been around since before I could (legally) buy a drink, it is a somewhat complex IC. Some models have problems with simulation convergence (you may get a "Timestep too small" error) while others are so simplified that they reveal none of the details you typically use simulation to investigate. If your problem falls into this category, try a different model. I believe there are several posted in the LTSpice User's Group on Yahoo.