A friend of mine asked me for help deciphering the less-than friendly instructions in the book "Plots Unlimited." It's a really great book, and I highly recommend it, but it can be frustrating if you don't know how to use it.
(The version we both have is the 4th edition, 1999, ISBN: 0-9627476-0-2, so if you have a different edition your page numbers might not match up.)
Remember the "choose your own adventure" books from when you were a kid? This is sort of like that on a larger scale, except there are no "wrong" choices to get you killed. LOL (Unless you want to kill someone off.)
Okay, with PU (haha!) there are two main ways to approach story generation if you're doing it from scratch, either with conflict/situation, or characters. Regardless of where you start, it's sort of like a Chinese menu. Some from column a, some from column b.
For sake of argument, let's do the first one so it's easier to follow. LOL
Pg. 16 starts the MasterPlots With Interchangeable Clauses chart.
Let's do "A" Clauses 1. A Person in Love.
Combined with "B" Clauses (1) Undertaking a difficult venture when promised a reward for high achievement,
"C" Clauses (1) Suffers a heavy penalty as the result of an ill-fated venture.
Okay, so see where in B and C they both have a (1)? Go to page 21 where they have the index of clauses.
See where at the top left of the page it says: (1) Romance 110
That (1) corresponds with the (1) in both our B and C clauses.
So now turn to page 38.
See the black 110 in the upper left column?
(1) Undertaking a Difficult Venture When Promised a Reward for High Achievement.
(1a, g) (4b) (22a)
Jack is in love with Carol, who will be permitted to marry him if Jack:  succeeds in a difficult venture (209)...
Okay, with me so far?
In the first part, had I chosen different numbers for B and C instead of using (1) for each, I could look them up separately.
Say for my "C" Clause on page 16 I had instead picked (2) Emerges happily from a serious involvement
I would turn to the index on pg. 21, and where it says (2) Romance 16, 119, 217 I could have looked those up instead for those sections.
With me so far?
Remember, these are suggestions for ways to take your story. You don't have to use all of them.
Okay, back to our original premise.
On number 110, where it starts out with (1a, g) (4b) (22a)
Those are potential scenarios leading up to the plot point if you want to use any of them to flesh the story out more.
For example, 1a is on page 25.
(50) Compelled by a Special Motive to Behave Underhandedly
(a) (112) (117)(148)(656)
Jack, poor, loves wealthy Carol * Jack, in love with wealthy Carol, pretends to be rich **... (etc.)
Those asterisks show where you can use some, all, or none of those extra points in your skeleton of a story. Again, there are reference numbers before and after where you can build your story backward or forward.
You can go as far or as shallow as you want with all of these.
Okay, let's look at the character section.
Starting on page 183 is a character combination index. It has character names and relations, and sometimes an X (which is an object).
Okay, for example, a menage of two guys and a girl would line up with page 278 Jack, Carol, Pete - Jack, Carol, and male friend (pg 185)
So turn to pg. 278. You'll see the black heading Jack, Carol, and Pete with options below it.
Jack, bringing news of the death of his friend... (yadda yadda) ...who have never seen Jack or Pete 5
That 5 refers back to pg. 26. where you see the black heading 5.
In the character section, any of them that start with a number in ( ) that's referring you back to the corresponding number in the index starting on pg 21. So you can fill in that blurb from the front end, or from the back end from the numbers following the blurb.
You just keep going back and forth in the little snippets as you build your plot, as much or as little as you need to. If you have a partial plot, find the things that correspond best with YOUR story that you have now, and build from that point on, if that makes sense?
Hope this helps!