If on captain sim 757 you encounter a blank grey box where the FMC, etc should be try this;

The Steam Edition of FSX uses a custom version of simconnect.

Many addons use simconnect for displaying objects and if you are having problems with aircraft display in the Steam Edition you may be able to fix them by installing the original FSX simconnect

In your Steam FSX folder (usually in Program Files (x86)\Steam\steamapps\common\FSX\ ) look in SDK\Core Utilities Kit\SimConnect SDK\LegacyInterfaces\FSX-XPACK\

You should see SimConnect.msi in that folder. Run this to install the original (legacy) version of simconnect

Happy flying  :)