Hi Bob,
I've been looking at this problem and it's fixed for the next release.
There is a workaround in the meantime. The crosshairs for objects/shaders have kind of an invisible selection zone around them. This is to make it easier to select items, otherwise you would need to be very precise with the mouse. What is happening when you try to select a population instance that is over or near another objects crosshairs is that the selection zone around the crosshairs is preventing selection of the population instance. The easiest way to way to work around this is to zoom in on the population instance. This makes the crosshairs selection zone smaller relative to the population instance, meaning it will be easier to select the instance.
The quick summary is to zoom in on the population instance and try selecting it further from the crosshairs of the other object.
Like I say, this is fixed in the next release.
Regards,
Jo