In the case of importing a route the problem is that there are not times for the points like there are with a track.
To modify the file so that it can be opened in BaseCamp open the GPX file in a text editor, Text Edit will work on a mac. In the text editor you need to remove all instances of "<time>(null)</time>", if there are more than a few points in the route the easiest way to do that is to choose "Find and Replace" from the edit menu, enter <time>(null)</time> in the find field, leave the replace field blank, and click replace all. Save the file, and then it should open in BaseCamp.
Be careful not to change anything else in the GPX file, BaseCamp is very sensitive to any formatting errors.
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Jesse Crocker