Easiest program to convert DVD for plex?

The easiest program to rip a DVD for use in PLEX would be "MakeMKV"  Just Google search for it.  It's a great program to easily Rip DVD's, HD, DVD's and Blu-Ray's to a MKV file.  Which is just a container for MP4 type files.  

You can leave it at that or better yet,m use a program called Handbreak and you can compact the file even smaller.  Doing this takes a lot more time then just ripping the disc.

Make sure you name files correctly.  Generally for a Movie File, I have all my movies in their own Drawer.  So for a Example, I have a Drawer "Movies" and in that Movies Drawer I have one called "Alien (1979)"  Then inside that drawer is the Movie "Alien (1979) (1080p)"  It's a MKV file as are most of my movies.  I have Drawer for TV and Music content also.  

In Plex, You should have created a Library.  Mine is just simplely called MOVIES.  The Library Type should be Movies.  You then want to add a folder or folders where your movies are located.  Since PLEX is running directly from my NAS, the location of my movie files are in "/data/Media/Movies".  PLEX will search there, find "Alien (1979)" drawer, look in that and find the Video file.  Since all my movies have a release date, it helps with PLEX getting the correct Data.  After there there are movies that a remakes of older movies.  So data could end up wrong and then you would have to manually correct the data PLEX got wrong.  This was it's almost always right.   If there's a Movie with "The" at the begining like "The Cannonball Run", you should name it "Cannonball Run, The) or in my case "Cannonball Run, The (1981)"

For TV Shows, you want Name, Season and then Eposode Number. So say you have "Homeland"  So you would call it "Homeland S01E01"  That is Season 1 Eposode 1.   So you go on to S01E02, S01E03, up to S01E24 or whatever the last one is.  Then next season, it's S02E01 and so on and so on. It's really pretty simple.  Sometimes 2 episodes are combined into 1.  So you would do something like S02E02-03.

Again I keep each TV show in it's own folder.  In fact I keep each season in it's own folder in that folder.   PLEX has no problem finding any of it.  

Plex should find a mp4 file just fine and know what to do with it.  So my guess is you don't have PLEX setup to find it in the correct directory you have the file in.  I've been using PLEX for years, so it  seems pretty simple to me.  You'll figure it out.  You can also get help here at the Plex forums!!!  https://forums.plex.tv/

Since you're here, I assume you're running PLEX on your NAS, in which case you may not have your directory setup correctly.  So again I'musing "/data/Media/Movies" Without the quotes in PLEX of course.  So Movies is where most of my Movies are located in that Drawer, anf in there is Alien (1979) drawer and in side that drawer is the movie Alien.    On my 516 NAS my Media stuff is located in /data/media/   though just /Media/Movies/ seems to also work.   So I think this is the issue you may be having.   If you have the directory wrong, PLEX will never find anything.