So at level 12 the Mesmerist gains access to Masterful Tricks. The text states:
At 12th level, a mesmerist can choose one of the following masterful tricks in place of a mesmerist trick. He gains an additional masterful trick every 2 levels thereafter.
It seems straightforward - like most classes, you take your "advanced" option or your normal options - but read that second line. Ever 2 levels you gain an additional masterful trick. For some reason, this line doesn't include the same wording as the first - it doesn't say you can choose from them instead of from the mesmerist tricks, it says you gain "additional masterful tricks". From a strict RAW perspective it appears to be saying you gain a Mesmerist Trick and a Masterful Trick every 2 levels starting at Level 14 rather than one or the other.
I don't think this was the developer's intent, but I do think the wording is misleading. Some specification would be very helpful.
It's pretty clear they meant "He can gain an additional masterful trick every 2 levels thereafter."
If you could take both they'd both be listed on Table 1-4: Mesmerist, but they're not. They only place masterful tricks shows up (next to mesmerist trick) is 12th level.
So it's a modification/access to masterful tricks from the usual mesmerist tricks progression, not additional tricks.