I was hoping someone might have replied to this query as I don’t quite follow what you are trying to achieve and I don’t use Windows these days.
I use an iMac and have an eTrex30.
Here is a bit of info that may help…I think you should expand your query and tell us much more explicitly and expansively what you are having a problem with. I don’t know how to convert between formats as I just download the types I need from the providers in the link below.
If you just want to put an OSM of Canada on your Dakota10, then you only need to download a .img file and copy it to the Garmin directory on the Dakota.
Here is a list of providers http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/Download
Lambertus provides a good service and regular updates but any of the others are fine too. You will be sent an email with a link to download the .img file.
The Javawa GMTK and the Javawa device manager programs mentioned in the link above are useful.
Alternatively, you can use Custom Maps (Garmin custom raster maps) http://www.garmin.com/us/products/onthetrail/custommaps#fragment-2 which are just a .jpeg converted to a geo-referenced .kmz map which are ok for small areas if you can find an jpeg image you really like. (These are not the same as zipped Google kml files with the kmz extension)
Alternatively, you can use the Birdseye imagery supplied by Garmin. The price seems ok in the USA but where I live is prohibitively expensive.
I think in the USA and maybe Canada you can get free 24k topo maps for free which is amazingly good value considering nothing is available for free in most countries.
I use another source for maps. I find openmapchest.org has more detail… (eg. on a recent trek i found the trail intersected with an abandoned railway… It didn’t show in openstreetmap.nl’s map). which makes sense because most people use openstreet map for (car) navigation, but I can use it for both car navigation and other activities without multiple maps.
My first thought was that I had to create a map from scratch, but the instructions are a bit confusing. I was hoping for a nationwide map so i don’t have multiple selections, and its easier to keep up-to-date.
The dakota has limited memory & no micro/sd slot; so its nice being able to select part i need. I also transfered part of garmin’s topographic maps… which are out of date but helpful because they show elevation. I have windows 7 installed on bootcamp/macmini but I use osx (basecamp) for plotting routes, creating tracks/waypoints.