I'm going to give you another tool to download (it will also root your phone in one click of the bat file). Ok. This is what I did for my htc hero s (same phone as the htc evo design 4G). I did not do the fastboot command but flash the img file in hboot. Since the design doesn't work that way here is a work around. I can't guarantee this will work or for the safety of your phone so do this at your own risk. All files except recovery are attached as you said you already have the recovery downloaded. credit goes to 2fast4u88 for the recovery and one click root tool, android for the access to the fastboot file, and Lou Ziffer for the fastboot work around to flash the recovery. I'm assuming that you do not have the android sdk and fastboot already and is the reason for this mangled up guide and workaround.
Download OneClickRoot (v1.1)
(file is attached to this message)
[Tool] One Click Perm-Root & Busybox Installer • Htc Hero S • 2FastRoms
Windows htc drivers
http://dl.dropbox.com/u/37835945/HTCDriver3.0.0.007.exe.zip.
Fastboot.exe
(file is attached to this message)
Unzip files and make sure htc sync is uninstalled before you install the drivers.
Install the drivers.
After the OneClickRoot_1.1 is downloaded and unzipped we will use the one click script to place a few files in its tools folder. This will give you access to adb and fastboot.
On your hard drive create a new folder called Android and move the "tools" folder from OneClickRoot folder into it (for example C:\Android/ - so it will look like this: C:\Android/tools). The tools folder will have the files adb.exe and AdbWinApi.dll and you will place the fastboot.exe file posted below into this same tools folder as adb.exe, AdbWinApi.dll . This should allow you to do the commands below.
Take the recovery img zip you downloaded for the custom recovery, open with 7zip, take the recovery img and put in the same folder as fastboot.exe file is located in the C:Android/tools/ (same tools folder you placed the fastboot.exe in above).
Plug your phone in to the computer and make sure it's set to charge only and reboot to the hboot and then select fastboot.
Open the terminal (or cmd prompt on windows) then navigate to where you moved the tools folder C:\Android/tools to get its path to type into the command prompt. CD the tools folder. (For Example: If you created the folder in C:\Android, then you would type in Command Prompt:
"cd c:\Android/tools" (without the quotes). Hit enter.
It should return a line and show that you are in pointing the command prompt to the the correct place. If that is correct, then type in the command prompt
"fastboot flash recovery recovery.img" (again without quotes).
It will do its thing and since I did not flash my recovery in fastboot I'm not sure of what the command prompt or your phone will do after you type in the fastboot commands, but once this is done you should have a working cwm recovery. Please make sure that if you placed the recovery img file on the root of your sd card (not in my instruction) that you move it from there (delete or move to your computer for safe keeping) because it will keep your phone from booting if left there. I hope this works for you.