*Slight Spoiler Warning*
To get access to a Grinder, you need to complete the mission "Grinders". This is given to you by Janey Springs after you complete your fifth story mission "A New Direction" (which gives you an achievement).
When looking for the Grinder Buggy, it may not be in the highlighted area on your map in Triton Flats. It may be roaming somewhere in the area (mine was fighting a Shuggurath close by).
The second grinder is on the roof of the Tram Station if you're having difficulty finding it in Stanton's Liver.
Upon returning to Concordia, build the Grinder and set in 3 items of your choice or the 3 guns Janey gives you. On a side note, there are many limitations as to what you can grind, and figuring out definitive recipes is still under way by the entire borderlands community.