I have coordinates in the form eg. Easting 56,166 Northing 761,040. These are not compliant with the format options on my Garmin. I am wondering how to convert them or how to enter them into the coordinates search?
You can get coordinates from Google maps using this Coordinates Finder.
In Google Earth preferences you can play around with the different formats to find the location using one format then mark the location and change to degrees.minutes.seconds which is what you need for most GPS units.
But... you may have to just drop the very last digit off the end when doing the input into your unit. Don't worry though, you'll still be within a few metres.