If the DeskCycle was pedalled with a loose pedal it can strip the threads on the crank arm. This can happen if the pedal wasn't fully tightened in the first place or allowed to come loose after installation.
It's always a good idea to check the pedals after they've been installed.
Another reason why the crank arm could be stripped is by trying to install the left pedal into the right side or the right pedal into the left. Forcing a thread into the wrong arm will damage the threads.
The third common cause of damaged crank arm threads is from cross-threading. This is when the pedal was forced into place but was not properly squarely seated into the threads.
If you find your pedal has backed out after installation, try a bit of locktite on the threads and tighten again.
For repair simply take your new crank arm and DeskCycle to the bike shop. Installation of new crank arms require a special tool that your local bike shop will have.
Improper installation is not covered by the manufactures warranty.