How can I hook up multiple MMA8451 accelerometers, (I2C protocol) using one Arduino Mega?
Brief explanation: When you talk to the slave device through I2C you start with (slave) device address. Then you send the message. It is like calling your colleague in an office with many other people. You say: "John", and when he is aware you continue with message, like "let's go for coffee".  

This arrangement allows for adding up to 127 devices on one I2C bus. Each device must have different address (sorry, you cannot have two John's in the office, it will cause conflict). Generally two different type of devices will have different I2C addresses, but it may happen you need to use two (or more) of the same (like in your example). In this case, see below.   

Many devices with I2C interface allows for address change. Using this facility you are able to communicate with many of the same devices through the same pair of cables.
You can check if your chosen accelerometer has possibility for address change: On picture below there is one with Address tab. When jumper is not fitted, you will get one address, when is fitted, the address of this device will be different, allowing of two of this accelerometers on one I2C bus. Unfortunately in this example there will be possibility of maximum of two of the same devices.
   

Of course there are devices with more than two possible addresses, you need to search for them.
And, yes, it is fine to use one Arduino to communicate with many I2C devices.  

EDIT:
Looking after some of Adafruit accelerometers, I have found this page: https://learn.adafruit.com/i2c-addresses  

And small off topic note: If you struggling on finding I2C with more than 2 addresses, check if it is easier to use SPI protocol. In this case you need (edited to correct bus lines names after user CrossRoads comment) MISO, MOSI, SCK and CS (chip select) wires. Chip select is individually connected to each slave device. This allows selecting particular device without addressing issue.