The red x's are probably there because it is a trial only.
I imagine Twixtor came with a manual, I would take a look at that. If not, then there are countless tutorials and resources on the internet that should get you up to speed.
As Dan said, you downloaded a demeo version. If you need the marks to go away, you buy the plug-in. And Dan is also correct that it comes with a documentation folder. RevisionFX also maintain a tutorial section plus you can no doubt find tons of slo-mo tutorials just by typing in a web search....