The POI files for the iGO 8.3 version I'm using have a .poi extension. The files from POIPlaza for iGO have either .asc or .kml

I put one of the .kml files in igo8/content/userdata/POI on the SD card and iGO hangs when the main screen says starting.

I tried putting one in igo8/content/POI and nothing new showed up in the POI management.

This is how to do it:

Save [Please Register or Login to download file] to your PC, next copy all saved KML files to igo8/content/userdata/poi (create userdata folder and poi folder if necessary)
iGO8 is slow to process new additions. It will take some time if you add many new KML files. You will also be asked to associate [Please Register or Login to download file] with new files. The most important thing with iGO8 is to be patient! At the first new start, iGO has to build some index files. All further starts will take a bit longer than without your own POI sets.

So when you say iGO hangs, it may be possible iGO is busy. Better do it adding small files one by one.

Also, some builds could prove to be more stable than others converting.