Not sure if there's an easy method of creating ITN files on a Mac, however, the ITN file is just as basic text file that you can easily create and load onto your Rider. Here's a sample I created for a ride-out recently, one of those coming along had a Rider so I converted my Garmin route for him, apparently it worked fine.
My comments appear in red and are not part of the file
---------------------------------------------------------
-333893|+5176720|Asda|4| start point
-347812|+5187182|Storey Arms|1|
-323470|+5188964|A40|1|
-316702|+5187235|A479|1|
-324593|+5200817|A4792|1|
-326985|+5203605|A470 Riverside|1|
-333859|+5208884|B4567|1|
-321650|+5221883|A442|1|
-333829|+5227489|A44|1|
-334332|+5243483|A4833|1|
-331972|+5247156|A4834|1|
-334375|+5249053|DolforRoadandA4|1|
-331795|+5250946|A483 Dolfor Road|1|
-344859|+5252567|A470|1|
-362760|+5261147|B4518|1|
-354455|+5245349|B45182|1|
-354636|+5242307|A4705|1|
-359714|+5239200|A4703|1|
-351066|+5230063|NorthStreet|1|
-350636|+5230217|Texaco|1|
-344395|+5221517|B4358|1|
-358009|+5214929|A4832|1|
-379869|+5199344|WestEndCafe|3| end point
---------------------------------------------------------
It's pretty simple, the coordinates are longitude and latitude to five decimal places but with the point removed and a + or - sign added as appropriate, a text label and an identifier (not sure what a 2 might do? but 4 is the start point, 3 the end point and 0 a via-point). You could look up the long/lat in Google Earth and create your own. I've just done a small experiment creating an ITN from a Google Earth KML file for a small route along the Gorge du Verdon as follows:
1/ In Google Earth use the path tool to create a path, adding nodes where the route could vary (don't use too many). This path will be point to point rather than following the roads.
2/ Save as a KML file to your PC.
3/ Open the KML file in a text editor, the last parameter is a string of coordinates, copy that string and paste into a blank text document.
4/ Format the string like the example below:
Before
6.254430532848829,43.80290616704094,0 6.260383834068763,43.7978181632409,0 6.272771558188695,43.78432430615938,0 6.290534812036015,43.77591866743446,0
After
+625443|+4380290|Start|4|
+626038|+4379781|via point 1|0|
+627277|+4378432|via point 2|0|
+629053|+4377591|End|3|
5/ Save as an ITN file, i.e. your_route_name.itn
It looks like the accuracy of the roads in Google Earth (the roads overlay) is less accurate than the real roads in the photos, I would suggest clicking on the latter when defining the nodes of your route path in GE.
I back converted this to a Garmin GPX and an MS Autoroute AXE and both worked OK in Mapsource and Autoroute respectively.
All in all a bit of an effort, unless someone can suggest an alternative bit of software. How about a windows emulator and using ITN Converter which works very well on the PC, it ties in with Google Maps to create ITN files amongst other formats.
