After creating the custom training plan, I`ve clicked on the 'Import into planned training' button. Then I was warned that any data I already have in this date range will be overwritten. I clicked confirm, but nothing shows up later under planned activity or training plans.
I confirmed that it is in your log. When viewing you log, in the black bar, hover over 'blog' or 'calendar' - select 'Planned'. This will allow you to see your training plan.