The watermark will not be visible to you until you download an asset in preview size. Once you download an asset, you'll notice it as a sheet overlay in the center of the asset. The file you choose as a watermark should be a sheer .png file. Note that this will not replace the standard watermark, which is the name of your account and the asset's 8-digit ID number.
You can access the uploader by going to Account > Organization Settings > Settings. Please note: You must be the owner of the account to access this feature.