With the 18F connected to the RS232 output of the MAX232 from the MICRO 50 board, you should insert a series limiting resistor (like shown in your PBP manual) for a direct RS232 serial connection, and use serin, serin2, debugin etc with inverted mode.

Some of the "A" version 14-bit core, and the majority of the 18F series will flat out refuse to work with direct RS232 level voltage applied to an input pin.

The best option is to just toss in another MAX232 converter. It can/will save you a ton of time & trouble. Some of the 14-bit "A" versions refuse to work even with the old series limiting resistor trick.

You can use the 18F's hardware USART with HSERIN/HSEROUT if you use another MAX232 IC (or equivalent inverter circuit) on the 18F side.

18F true mode: Can use any serial command or hardware USART
18F tx > ttl in >|max|> rs232 out >--> rs232 in >|max|> ttl out >--micro 50
18F rx < ttl out <|max|< rs232 in <--< rs232 out <|max|< ttl in <--micro 50

18F inverted mode: serin/serout, serin2/serout2
18F tx > -----------> rs232 in >|max|> ttl out >--micro 50
18F rx < --/\/\/\----< rs232 out <|max|< ttl in <--micro 50

Check the MICRO 50 board docs also to see if it requires handshaking. If you're not seeing anything output by the MICRO 50 board, it may be waiting for a control byte or handshaking signal before transmitting.

And as Charles already mentioned, be sure to use an oscillator that supports the data rate.