The installation program should do all the rest of the installation, and enable the map file for your device to use.
I have a screen shot below of what the screen will look like where you select the SD card.
In case you need a reference, I have a link to Amazon that show these cards and the going prices. By the way, don't waste any money on anything bigger than an 8GB card, as the price tends to go up after 8GB, and you will probably not need anything larger than 8GB.
Once you have that card INSTALLED in your device, you should be able to run the update program again with your device attached to the USB cable, and then "select" the SD card as the "destination" of the map files.
Again, this is because the map files have GROWN to such a large size (almost 2GB now), including all of the "points of interest" that have been included (hotels, restaurants, etc.).
Please give me a few minutes to find the details about your C340 files and which ones can be deleted.
I usually recommend a 4GB or 8GB capacity SD card, as they are very inexpensive and readily available at almost any store that carries GPS units or digital cameras, or many office supply stores.
I found the interface to select maps a bit cumbersome - some of the map regions were very small and not clearly labeled.
So if that is the case (the Garmin apps can see the device), then a program such as Map Install SHOULD also be able to see the SD card (when installed in the device) and should be able to put the maps there.
If that works, then it would just be a matter of "enabling" the map file inside the menu system of the unit, and turning OFF the maps that are there now.
Map Install did not show both the maps on the device and the downloaded 2013.30 maps on my computer - it's device finder found the Street Pilot and then showed a map diagram in which to select hundreds of map regions. I really do think that this is the way to go, so that you will be able to have the maps on a single SD card (assume 4GB card).
So, I do not see how to do the "turning OFF the maps there". One other note: When you downloaded the maps, did you select to install them on "device and computer" - similar to the screen shot I have below?