Basic Question On Coolant Covers

[markcheck]

New user, hope this isn't a repeat but tried to search and could not find this:

How do I add coolant to 2005 ls430? It seems to be in the very front center of engine bay but is covered by a small plastic cover (about 1-2 feet wide by 8 inches if I remember). Tried removing the screw (one of two) and lifting but I found it took some kind of screw-holder out of a hole that would not then go back in -- the bottom expanded as if it were designed to be removed and never go back in, and I had to squeeze separate pieces on the botttom of this screw-holder to try to get it back in the hole before I could put the screw back.

The manual says to add coolant when needed but I cannot even figure out how to get the the overflow tank, which for the diagram appears to be under this cover.

Also how can I find out what tool I need to remove main engine cover? I have always changed my own air filter but the manual doesn't even say how... I see two center bolts (not screws exactly) but don't know what kind of tool is needed to loosen or otherwise remove them. Thanks!