Did you power the car with the sensors removed?? If so you will need to go to a mechanic that can reset the electronics in the car. I did this in my wifes Honda when we removed the seat to install some GPS stuff under the seat. If the seat sensor is disconnected and the car is powered up this way the computer thinks there is a fault with the airbags seat sensor (indeed there is at this point as the sensor is not there). The computer stores this fault and will not allow the airbags system to operate until the code is cleared. Having a shop that has the proper tools to reset the computer will allow the computer to see that the sensor is indeed back in place and assuming you installed everything correctly it will again be happy and allow the airbags system to work again, and the light will be off.
If you did not power the car with things disconnected then you installed something improperly. If this is that case, even if you fix whatever it is you will still likely need to reset the computer in order to get everything working again.
Well if the airbag fault light is lit something is wrong. Reset procedures are listed in section RS of the shop manual (many many pages of data here). The precise code that the DCS is reading out should point in the right direction. Without at least the manual or even better, electronic diagnostic equipment for the DCS, I would not even attempt a repair beyond verifying good solid plug connections (disconnect and reconnect them again, perhaps one did not seat well) and resetting the DCS to see if the light reappears. This troubleshooting from what I just read in the manual can mostly be done without the electronics it may just take bit. Good luck.