What version of Premiere Elements are you using on Mac?
Export your Timeline content to a .mp4 file, using export settings which have been customized under the Advanced Button/Video Tab of the export's preset.
See Publish+Share/Computer/AVCHD and .mp4 category. The exported file is then uploaded to YouTube at the YouTube web site.
If you post the properties of your source video and project settings, I will suggest the settings for the Timeline content export to file saved to the computer
hard drive for upload to YouTube at the YouTube web site.