This applies to the 32 bit version of NPP, no?
It is currently only in the Plugins Admin for the 32-bit version.
However, that’s why @Meta-Chuh included the manual instructions as well; the link he pointed to will work for either 32-bit (…x86.zip) or 64-bit (…x64.zip)
Since Meta didn’t explain the manual install, but normally does, I get to do it this time :-)
To actually perform the manual install:
unzip the appropriate downloaded zipfile and open that containing folder (or have the zipfile open in windows explorer)
In Notepad++, use Plugins > Open Plugins Folder.
In the newly-opened plugins folder, add a subfolder HexEditor.
Copy the HexEditor.dll from the zipfile into the HexEditor subfolder… so it should be at ...\notepad++\plugins\HexEditor\HexEditor.dll
Exit all copies of Notepad++.
Run a fresh copy.
HexEditor should be available in the Plugins menu, and HexEditor.dll should be listed in the ? > Debug Info plugins list.
(Actually, I think he has those instructions, in a more generic form, in various places… but I forget where; I need to bookmark that for the future.)