After saving the desired Patch and the Channel Strip Settings for that Patch and loading them into a new MainStage Concert file you must first add all the included Samples and Sampler Instruments from the MainStage file you downloaded.
To do this you have to do is locate the Concert file in the folder you downloaded and right click on it. You should see an option that says "Show Package Contents", click this option.
Now you should be taken to a new window and there should be two folders named "Samples" and "Sampler Instruments". Copy these two folders, now navigate to the Sampler Instrument folder by following the path Users->Your Computer Name->Music->Audio Music Apps->Sampler Instruments.
Create a new folder and title it the name of the Saved Patch, for example "Flowers Pad". Open that folder and paste the Samples and Sampler Instrument folders in the new "Flowers Pad" folder.
Now when you open the EXS24 window for each instrument track inside of MainStage click on the missing file name and then "Refresh Menu". You should now be able to find and select the desired Sampler Instrument found under the "Flowers Pad" tab in the EXS24.