choose a different serial port other than the primary, or use SoftSerial. You are probably getting conflict with the USB/UART connection if you are trying to use main serial port, as it's also the connection to your computer and all that.
SoftwareSerial (and there's also NewSoftwareSerial, which fixes a few issues.. but anyway..) is what you want. You can define a serial port on any two digital pins you want, and that will be your LCD connection. Trying to piggyback serial connections ends poorly :)
OR, since you have a MEGA... use the secondary serial port rather than the primary.