Available in the new release (0.7.6) through menu "Image -> Deinterlace".

You have interlaced video when you can see the annoying scan lines artifacts.
For sports with great dynamics, it can quickly become a huge problem :

Hopefully one day the function will be implemented directly in Kinovea...
In the meantime, you can use VirtualDub and apply a deinterlacing filter on the videos. There are more advanced methods, but I find this one simple and efficient enough for my needs.
VirtualDub is an awesome open source tool for simple and fast video editing, you can download current version here: http://virtualdub.sourceforge.net/

1. Convert to .mpg or .avi if needed. (VirtualDub will only accept those formats.)
- If the file plays fine in Kinovea (except for the interlacing artifacts of course), you can try to save it back to AVI. (File => Save video or Key Images Data)
- Otherwise this depends on the format you're starting with. Your camcorder software suite may provide a tool to perform the conversion.
- For JVC .mod files, apparently you can simply rename them to .mpg. This will avoid loosing quality during the saving process.

2. Deinterlace
- Open your video in VirtualDub. Go to Video => Filters, click Add, choose the deinterlace line and OK.
- You use the "Blend Fields together" option choosen for a smoothing of the artifact.
- If you have a massive shift of an object between the half frames like in the above picture, you may want to try the "Duplicate Field 1" option.
- The right hand panel of VirtualDub should now have the deinterlaced video. (Left hand panel has your original video).

3. Reencode
- Click Video => Compression.
- Choose your preferred codec for reencoding. (I'd use XviD) This step depends on the video codecs installed on your computer.
- Do the same for audio if you want. (Audio => Compression) or choose No Audio.
- Choose File => Save as AVI and save to a new file.

You should now have a deinterlaced version of your video.
The best option is always to use the progressive mode of your camcorder if it has one.

Please report any issue in the comments.