Get your clip you want slo-mo, drag the twixtor effect on to it, set the appropriate properties in the twixtor window i.e. 50% time etc, then you have to drag the end of the clip on the time line to the appropriate size. In the case of your 50% slo-mo, 2 times as long.

I find that playback on the time line of twixtored clips is virtually non existent. you could either RAM preview it or just render out.

Look at the twixtor tutorials for how to avoid ghosting and trails.... a plain background usually helps, fast frame rate and not too much motion. I have used twixtor amazingly of my kids jumping in the air. Make sure I shoot really low looking up at them letting blue sky be the background. When they are at the top of their jump - ie. not moving much, I take a 0.1 second clip, apply 2% twixtor to it then drag the clip out 50 times as long (5 seconds). To have almost a still looking shot with a little motion can be a great effect.