robwlakes:
int RxPin = 8;means that instead of using the character/number "8" to represent the number of the pin in question, the label "RxPin" can be used instead. Change 8 to another number in that line and that number will be used by the rest of the program wherever RxPin is used. The comment added after the // is quite unhelpful. As jremington suggests the 433MhzRx's output is attached to pin 8 which is the input in this case.
Thanks for the help again.
I now have the WEMOS D1 Mini reading the data correctly and outputting the serial monitor.
Here are the changes I made to the code to get it to work.
// Libraries
// #include <SPI.h> Not needed as this is already included for ESP8266
#define MAX_BYTES 7
#define countof(x) (sizeof(x)/sizeof(x[0]))
// Interface Definitions
int RxPin = 13; // Receiver on interrupt GPIO 13 => that is pin D7