IMPORTANT: Following these instructions will probably result in the irreparable damage to your computer. Also, these chemicals will probably kill you in a few days, your home may be declared a superfund site and your family's hair will fall out, if your house doesn't burn down first. Basically, your life is in your own hands. I don't think you should do any of this. ( :
The best information I could find regarding the liquid, is that its green and may be propylene glycol. I found more reliable "looking" information that says its Delphi 151 Coolant Fluid.
Most importantly though, the system is sealed (or is supposed to be) so unless it shows signs of leaking, the level is fine. If there are signs of leaking (which is a problem with some models apparently) then you should stop using the machine and unplug it since it is now a safety hazard.
If it is leaking, you could do a couple things to fix it yourself. Remove the cooling tubes/heat sink to do this...
First, find the leak. You should see some corrosion around that area. Take some steel wool, and clean the area until it is bright and shiny.
Take a very, very small drill bit, about the size of this asterisk (*) and use it to drill the hole out. You are trying to remove any corrosion from the hole and open it up so that the original tube thickness is revealed.
Next, if you think there has been a significant fluid loss, you COULD use water, anti-freeze or the Delphi 151 fluid to fill it back up. Use a syringe to slowly add it, rotate the assembly around so most of the air comes up to the hole, fill some more, repeat. Do leave a little air space, you probably wont be able to help it.
Dry off any fluids that may have dripped and get ready for the fun part.
Once you have the area clean, the corrosion cleaned out of the hole and it filled with the fluid of your choice, get some acid flux and brush the flux around the hole.
Next, get out your super hot soldering iron and SILVER SOLDER. Apply some heat to the area surrounding the hole and fill it in with the solder.
Put it all back together and you are done. I hope it works for you. If this works, you will have saved yourself about $980. If you had to buy the tools, you probably only spent $150.