[was a comment - re-submitting as an answer] At this point the DE1-SoC is not supported out of the box. However, it should be possible to add it using the custom board API, which is illustrated here. Currently there are two Cyclone V SoC boards with predefined support.
Altera Cyclone V SoC Development Kit
It's not difficult to add support for a new board to the MathWorks hardware/software workflow for Altera/Intel SoC FPGAs. There's anexample that show how to do this. Also if you install theHDL Coder support package for Altera SoCs, you get access to all the board definition files, etc. for the SoCKit and Altera Cyclone V SoC Development Kit, which would help you add support for a new boards.