I am using a similar unit (except mine is a 5032 head unit with a Kenwood KNA-431 hide-away unit. but essentially the same type of thing ).
You will need for each POI: (lets just say Shell POI's as an example)
Alternatively to use the GPX file which may be richer info ??
Some units may use the wav file instead of the mp3 which may also require the sox.exe file. There are plenty of threads about this under the Garmin forum so give it a bash.
Also as a point to note. Download all you POI files to a directory and then use POILOader to create a single POI.GPI file. This can be loaded to the device either by using a SD card ( create a directory called \\garmin\POI and stick the POI.GPI file there .. when you boot the device it will copy the contexts of te POI.GPI file to internal memory overwriting any previously stored POI's .. So keep them in a single directoy on your PC/MAC so when you change one its easy to regenerate the whole POI.GPI file
There are other ways but this may be you going !!