I had the same questions when I first was put in a Yams game. Now I like it way more than regular Yahtzee. If you decide to try it again, here are answers to your questions. The plus minus is sort of the replacement for chance. You just have two throw away rolls and you score the differential between the points. So if you put a 20 on plus and have a 10 on minus, you will score 10 points. If you have a 20 on plus and a 23 on minus, you don't lose points, you will just score a 0.
Rigole is a roll that is a 4 of a kind with the 5th die equaling 7 when added to the value of the other 4. So a 11116, 66661, 22225, 55552, 33334 and 44443 are the combinations that make a Rigole.