Adding menu link to Table JX through joomla menu administration, doesn't work in v5 as it worked in previous versions. We will fix this till next release. However Table JX was first meant to be displayed within an article.
To achieve the same effect just select 'External Link' as menu type and write following in the 'Link' field:
?option=com_grid
If you click on the just created link item, you will get a list of all created grids as in previous versions.
