When you sideload, you copy the .prg to the device itself.
When you connect your watch to a PC, it will show up as a drive, and the .prg goes in \garmin\apps. The simplest way to do that in eclipse is to use "Build for device" in the Connect IQ menu in eclipse.
Look in the Programmer's Guilde that comes with the SDK, the section "Side Loading an App".
Also, look at the New Developer FAQ https://forums.garmin.com/showthread.php?339891-New-Developer-FAQ and https://forums.garmin.com/showthread.php?359183-New-Developer-FAQ-2
- In Eclipse, choose Connect IQ > Build for Device Wizard…
- In the Build for Device Tool window:
- Choose the project you wish to build
- Choose the target device
- Set an output location
- Click Finish
- Connect your device to the computer via USB (it should connect as mass storage and appear as a drive)
- Copy the resultant PRG file from the output location to /GARMIN/APPS on your device
- Disconnect your device; the app should be available for use