, I noticed that for the orbital parameters of your "Kepler-90.ssc" file you simply used the orbital elements given in exoplanet.eu. This is actually incorrect, as the planets won't pass in front of the star (at least when viewed from the Solar System).
To convert them to a Celestia's ecliptic system, use Grant Hutchinson's Star Orbits Spreadsheet
. Simply put in the star's coordinates, the orbital period (in years), eccentricity, inclination, argument of periastron, and epoch (in Besselian years).
For basically all planets, the position angle of ascending node (Ω) is not known. This can therefore be ignored, but both the Inclination and AscendingNode parameters that are calculated have to be used.
Also, this spreadsheet was meant for semi-major axes (a) to be in arcseconds. You can ignore this too, because exoplanet.eu already lists the semi-major axis in astronomical units and you can just copy that.
As for primary transit times, I forget how to implement that.
Luke (copied from this message earlier