This same issue has happened to me. First you need to download and extract the drivers from DeLorme's website here:
Extract the files into some directory.. (IE: c:\EMateSetup)
Next open up the Device Manager, you can quickly go to this by launching "devmgmt.msc" from Start->Run.
When you plug the earthmate in it should show up in Windows Device Manager under Human Interface Devices. It will probably be listed as "USB Human Interface Device", if you have a mouse or other HID devices they will also show up as this... right click on the first device with the previously listed name "USB Human Interface Device" and then go to update driver. Next choose "Install from a list or specific location (Advanced)" and click next. Check the radio button "Search for the best driver in these locations" and also check "Include this location in the search:"... now enter in the directory where you extracted DeLorme's usb2serial driver then click next. (IE: C:\EMATESETUP)
If the first listed device is not the Earthmate, the above process will yield nothing, try the above with the next listed HID device. When it successfully finds the Earthmate, say yes to the dialog asking you if you are sure you want to install an unsigned driver. You may or may not get this.
Finally... you may have to restart to see if it worked or not, it should be visible and configurable in the device manager under "Ports (COM & LPT)".
I hope that helps.