I assume you are referring to a Team KFWB caches (or caches based on their caches), since you are from the Island.
In geocaching speak, round earth refers to the default coordinate system that your GPS is normally set to. This is latitude, longitude based on a specific datum and commonly just referred to as WGS84 or NAD83. The units usually are expressed in degrees, minutes and decimal minutes and is generally referred to a geographic coordinate system.
Flat earth on the other hand, refers to the Universal Transverse Mercator (UTM) projection. This is a plane coordinate system as opposed to an geographic coord system and over short distances compensates for the curvature of the earth. The coordinates are referenced in metres north of the equator relative to 123rd meridian for most of BC.
You can easily switch coordinate systems in your GPS or mapping software to enter values in either system, then switch back to complete the conversion. There are many online geographic to UTM convertors as well, but doing it in you GPS is just as easy. Cache pages on geocachhing.com show the cache coordinates in both systems, right at the top of the page.
So, if the cache page says assume round earth, just use what ever tools you would use to project a waypoint (like the handy widget built right into the left sidebar right here at at the BCGA website) or right in your GPS, and you should be set to go.
BTW: This is an overly simplified explanation... please do not get me started.