To give a user a pre-existing device you want to Assign the device. Un-assigned devices are listed on the Device pages.
To assign a device to a user click on the Assign button.
Next select the type of Device you wish to Assign from the drop down menu and enter the device number or partial number and it will find what devices of that type are In Stock. Unless the device is in a different Cost Centre to the user leave Inherit Parent Cost Centre checked. Effective date is when this device started being used by this user (if needed please correct). Then click Add Device.
The device is now assigned to the user.