The Remote Desktop Connection Client for Mac won't make a remote connection to Windows 2000 Pro. Only the following versions of Windows support Terminal Services or Remote Desktop connections:
Windows XP Professional
Windows Server 2003
Windows 2000 Server
Windows 2000 Advanced Server
Windows 2000 Datacenter Server
Windows NT Server 4.0, Terminal Server Edition
Also be careful not to confuse Remote Desktop or Terminal Services (Remote Desktop is a form of Terminal Services for XP Pro) with Remote Access. Remote Access has to do with setting up a VPN that remote users can dial into to get access the to local network at the office.
The best way I know of to accomplish what you want is to use VNC. VNC stands for Virtual Network Computing and is designed from the ground up for one platform (ie Mac) to remotely control another computer of a different platform (ie Windows 2000 Pro).
The following links are to different Mac VNC clients:
Chicken of the VNC:
http://sourceforge.net/projects/cotvnc/
VNCDimension:
http://www.mdimension.com/page/VNCDimension?appNum=1&wosid=tMdCG7ccnut52cRX2IgtVM
VNCThing:
http://www.redstonesoftware.com/Download/VNCThing2.2.sit
Any one of the above should help you control a Windows 2000 machine.
If you want to do the reverse, control a Mac with a Windows machine then you need a Mac VNC server. This is the only one I know of for Mac:
http://www.redstonesoftware.com/vnc.html
For Windows I always use TightVNC which includes both a client and a server:
http://www.tightvnc.com/download.html
You'll want to set up your Windows machine as the VNC server and basic instructions on how to do this are here:
http://www.tightvnc.com/winst.html