Q: how do I convert a ts_folder to mpeg4 (or another format) for use in iMovie?

Use MPEG Streamclip to rip to QuickTime using the Apple Intermediate Codec.

do i need to download sw to convert my videos?

You need to convert the VOB files in the TS-Folder of the DVD back to DV which iMovie is designed to handle. For that you need mpegStreamclip:

http://www.squared5.com/svideo/mpeg-streamclip-mac.html

which is free, but you must also have the  Apple mpeg2 plugin :

http://store.apple.com/us/product/D2187Z/A/quicktime-mpeg-2-playback-component-f or-mac-os-x

(unless you are running Lion in which case see below))

which is a mere $20.

Another possibility is to use DVDxDV:

http://www.dvdxdv.com/NewFolderLookSite/Products/DVDxDV.overview.htm

which costs $25.

For the benefit of others who may read this thread:

Obviously the foregoing only applies to DVDs you have made yourself, or other home-made DVDs that have been given to you. It will NOT work on copy-protected commercial DVDs, which in any case would be illegal.

And from the TOU of these forums:

Keep within the Law

1. No material may be submitted that is intended to promote or commit an illegal act.
3. Do not submit software or descriptions of processes that break or otherwise ‘work around’ digital rights management software or hardware. This includes conversations about ‘ripping’ DVDs or working around FairPlay software used on the iTunes Store.

If you are running Lion or later:

From the MPEG Streamclip homepage

The installer of the MPEG-2 Playback Component may refuse to install the component in Lion. Apple states the component is unnecessary in Lion onwards, however MPEG Streamclip still needs it. See this:

http://support.apple.com/kb/HT3381

To install the component in Lion, please download MPEG Streamclip 1.9.3b7 beta above; inside the disk image you will find the Utility MPEG2 Component Lion: use it to install the MPEG-2 Playback Component in Lion. The original installer's disk image (QuickTimeMPEG2.dmg) is required.

The current versions of MPEG Streamclip cannot take advantage of the built-in MPEG-2 functionality of Lion. For MPEG-2 files you still need to install the QuickTime MPEG-2 Playback Component, which is not preinstalled in Lion. (The same applies to Mountain Lion and Mavericks even though they have it preinstalled.) You don't have to install QuickTime 7.