There are at least three solutions:
1) OPC
Matlab -> OPC-Toolbox (as OPC-Client) -> OPC-Server -> PLC
If you are using Plcsim, you need an extra glue program called Nettoplcsim:
Matlab -> OPC-Toolbox (as OPC-Client) -> OPC-Server -> (nettoplcsim -> PLCsim)
OPC-Servers for S7 are not free of charge, you have to buy one.
2) Using Plcsim S7ProSim Interface
This is the official interface to S7-Plcsim. This is delivered as COM-Object. If Matlab supports COM objects, you can directly call this objects.
If not, Matlab supports to load dll-libraries. So you can encapsulate the COM-Object into a flat C-dll (have done this before).
The documentation of this COM object you can find in your Step7 installation directory (e.g. C:\Program Files\Siemens\Plcsim\s7manual\s7wsi).
3) Using free S7driver dll and use this with loadlibrary
There are free S7 communication libraries like libnodave or snap7. These are delivered as dll, which you could use from Matlab with loadlibrary.
If you want to use it with Plcsim, you need also nettoplcsim as glue program.
The data flow then would be:
Matlab -> loadlibrary (e.g. libnodave, snap7) -> (nettoplcsim - PLCsim)