First, make sure that your micro:bit is plugged into the MakerBit board with the micro:bit LEDs facing upwards, and also make sure that the MakerBit board is plugged in using either the USB cable or a 9V battery.
If you are using the ribbon cable with the individual sockets, it’s possible that you plugged in the LED with the wrong legs. The short leg of the LED should be inserted into the socket with the small triangle. When viewed from the side where you can see the metal, that will always be the socket on the left, connected to the brown, orange, green purple, or white wire of the pair of wires going to your LED.
Also check the wires of the LEDS, photocells, or any other component to make sure that they’re not crossed or twisted together. You can also try taking out the LED and plugging in a different one. Failures of LEDs are rare, but it can happen.
If it seems that some of the pixels of the built-in LED of the micro:bit aren’t lighting up when connected to the MakerBit, it could be because the blue trimmers for A3 and A4 aren’t turned fully clockwise. The LED inputs are connected to various pins of the micro:bit, including the analog inputs, and the blue trimmers become part of these connections when the micro:bit it plugged into the MakerBit board. Just turn the A3 and A4 trimmers fully clockwise, and the micro:bit LED pixels will light up as expected.