The Byte Packing block should exist on both Linux and Windows installations. It should be located in the Simulink Real-Time > Utilites library. You can access this directly by typing in "xpcutilitieslib" at the command line. If the block exists there, you can simply replace the missing block in your model.
If the block does not exist in your Simulink Library browser, you can try refreshing the toolbox information by entering the command:
If you have Simulink customizations, you may need to also issue the command: