You must have made a mistake uninstalling, especially if you were using the Steam version and you have or HAD the regular FSX installed at some time so now, your registry keys are wrong.
If you use the Steam version and have or HAD the regular FSX too, the correct reinstall procedure is:
1) Uninstall regular FSX
2) Uninstall FSX:Steam Edition
3) Reinstall FSX:Steam Edition
4) RUN IT ONCE, so the Steam interface can set your registry keys correct for a "Single mode" operation (FSX:SE without FSX installed)
5) THEN you can start reinstalling your addons.
If you are unable to uninstall the regular FSX, it's best if you REMOVE ALL the FSX registry keys, then proceed from Step #2