Connecting SDI and SDO to a single DATA pin
Based on the picture, it seems you have a breakout board/module for the MMA7455, by Parallex? The chip itself allows for both 4 wire and 3 wire spi, but that module only has the 3 wire mode enabled.

But that's not really a problem. The thing about 3 wire mode is that it is half duplex, only transmits one way at a time. And this device is a slave device, it does not send arbitrary data at all times. By hooking up sdi and sdo to the same pin, you just have release the sdo pin (tristate/input mode) when you expect to read data. You Control the Clock so you control when and how fast the accelerometer sends data. You assert the cs pin, you send the register address with the read bit, then before you start clocking in the data, you release the SDO pin. You clear your buffer, then you clock the data in. If you're not ready, don't tick the spi clock.

What you want/need to do is also take a look at the chip's datasheet, not just the module's datasheet.