-- Just to be clear -- the Input/Output manual (on p. 136) is giving the format for the text-file format that the SWAT executable will read from the txtinout folder. It is generally far simpler to enter the data using ArcSWAT, which will read the data from a friendlier format and write the text files for you. But, this would probably require re-writing all other input files as well, which perhaps you don't want to do.
-- As a work-around, you could copy your project, use ArcSWAT to write the new precip files, and copy the new ones from the new txtinout folder to your original txtinout folder. (And then just trash your project copy -- you don't need anything else from it.)
-- Finally -- you've got example pcp files in your txtinout folder already, which should make the format apparent, no?
-- I avoid generated precip values at all costs (I'll fill missing gaps from nearby stations, if at all possible), but if you do use them, don't expect to calibrate to them, unless you've got a long enough time window (a year? longer?) for average values to be meaningful.
-- Jim