Windows
Installing a Single-Machine License on Windows: Universal Password Version
Installing a Single-Machine License on Windows: Machine Specific Version
Installing a Network Client License on Windows

Mac OS X
Installing a Single-Machine License on Mac OS X: Universal Password Version
Installing a Single-Machine License on Mac OS X: Machine Specific Version
Installing a Network Client License on Mac OS X

Linux/Unix
Installing a Single-Machine License on Linux or Unix: Machine Specific Password Version
Installing a Network Client License on Linux or Unix

Mathematica License Manager (MathLM)
Installing MathLM

Already installed Mathematica 7?
Find your Mathematica 7 License Number
Find your Mathematica 7 Password