Where do I top up my Toyota Prius Hybrid Transmission

Check your hybrid drive coolant reservoir for coolant level. top off if needed using only Toyota coolant. Also common for the auxiliary water pump to fail. I think there is a recall free replacement. Check with your dealer if your car is included in recall.

Some do NOT have filler cap or dipstick.
Need to see tranny shop.
Is there drive,2,low on transmission shifter indicator?
Some have CVT trannys and takes special fluid for CVT\'S.

how can I top up the water in my toyota prius radiator

The following is from used-cars website edmunds.com ... Refer to this forum for your answer : http://answers.edmunds.com/question-Looking-to-download-an-Owners-Manual-for-an-08-Toyota-Prius-26439.aspx

Check there staring Shaft and turing point it might be lose...

I don't know about hybrids, but on most vechiles, you check power steering fluid with engine running.

could be as simple as you need power steering fluid, which intels you may have a leak, or it could be a bad pump. add fluid, see how long it stays in there, check every day andmore. if it doesnt last a week of driving then you have a leak somewhere. if it does but the problem persits, its the pump

Velocity sensor or somtimes called speed sensor.   They communicate the transmission to the engine's computer so they know when to shift up or down on the gears. 
Ps: don't break them. Good luck and hope this help's

Seems to be alot of this problem mentioned on this forum regarding prius warning lights.
Don't worry yet might just be a service reminder.
Google " 2005 prius warning light " the first page will show a link to toyotapartsandservice.com where you can find the owners manual that covers all the warning lights.

Here is the answer that you are looking for: Never.
The auto trans fluid in a Prius is what we call "World Spec", and never needs to be changed. Why? Because the Prius trans fluid is not used for hydraulic pressure or cooling the ATF fluid due to the lack of the aforementioned hydraulic pressure. All that the ATF does is perform a "lubrication" function. As long as there is no contamination of the fluid (such as being in a flood), do not bother with it.
Regards
Toyota Ed
Prius Master Tech