“These are short for: Decimal, Hexadecimal, Octal and Binary.”
(never use a colon to introduce a list without “the following…” or something similar

“The Tape button shows you a drawer with the history: all the calculations you’ve made since you turned it on.”
(sketchy colon use – clause after the colon is not independent)

“Now, one of the most important buttons, the RPN button.”
(improper comma use – HERE is where you want a colon (”Now, one of the most important buttons: the RPN button.”) )

“Most of us, though, might not want to switch, because regular notation is just fine for us, and you don’t have to. ”
(most of us/you don’t have to — sudden switch in subject from plural first person to singular second person)

“Along with the built-in ones, you can define your own constants. To define your constants, go to Constants>User>Edit User Constants… You may want to take a constant from the built-in ones and put it in your user constants, because user constants have shortcuts. ”
(extremely clunky – could be combined into fewer, smoother sentences: “You can also define your own constants in addition to the built in ones. To do so, go to Constants>User>Edit User Constants… This feature is helpful as user constants, even if they are built-in, are automatically assigned keyboard shortcuts.

“You heard me right, a conversion tool built-in to a calculator!”
(incorrect use of built-in; should be built into. Also has the tone of a cheesy advertisement)

“you can have a number inputed”
(you can input a number)

“There are 4 themes and 4 horizontal layouts and 5 vertical layouts.”
(There are four themes, four horizontal layouts, and five vertical layouts.)

“PCalc for the mac is amazing and the iPhone version is definitely no exception”
(exception to what? The mac version being amazing? how about “no different”)

“you can really see the time put in to making this app.”
(put into)

You can download a trial of the Mac version, but, there are no App Store trials yet.
(incorrect comma use: “…Mac version, but there are no App Store trials yet.”

“You heard me right, a conversion tool built-in to a calculator!”
(incorrect use of built-in; should be built into. Also has the tone of a cheesy advertisement)

“you can have a number inputed”
(you can input a number)

“There are 4 themes and 4 horizontal layouts and 5 vertical layouts.”
(There are four themes, four horizontal layouts, and five vertical layouts.)

“PCalc for the mac is amazing and the iPhone version is definitely no exception”
(exception to what? The mac version being amazing? how about “no different”)

“you can really see the time put in to making this app.”
(put into)

You can download a trial of the Mac version, but, there are no App Store trials yet.
(incorrect comma use: “…Mac version, but there are no App Store trials yet.”