The HI-TECH C Compiler has been discontinued and is no longer supported. Some years ago Microchip purchased HI-TECH and has incorporated much of their technology into the new families of MPLAB® XC Compilers. Older versions of MPLAB 8.xx do include the HI-TECH Compiler as part of the installation. There is also a HI-TECH software archive for the compiler.
The HI-TECH compiler is still supported by the book "Beginner's Guide to Embedded C Programming" and you can download the MPLAB version with included HI-TECH C Compiler for the book.
The link to the HI-TECH archive is to provide backward compatibility for those projects that cannot be migrated and to give you time to migrate your systems over to the current MPLAB XC equivalent compilers. The HI-TECH C Compiler for PIC10/12/16 MCUs has essentially been replaced by the MPLAB XC8 compiler, which supports all the 8-bit PIC microcontrollers. You can download the latest MPLAB X IDE and XC8 compiler for free.