"after it installs" - where did it install to? Are you distinguishing between the downloaded installer
and the installed Mathematica?
Quick suggestion: http://support.wolfram.com/kb/3274
Did the installation go normally? See http://support.wolfram.com/kb/10650
Suggestions will have to be operating-system dependent.
On Windows:
- Look in Start menu - All Programs for Wolfram Mathematica. Can you launch it from there?
- Does holding down the SHIFT and CONTROL keys as you try to start Mathematica make a difference?
- The Mathematica Kernel should be in
Start menu - All Programs - Wolfram Mathematica - Mathematica Kernel.
Can you start that?
On Macintosh:
- Was there a problem unpacking the downloaded .dmg file?
- In the Applications folder, does Mathematica.app show up?
- Try the Mathematica Kernel:
- Open the Applications folder,
- hold down the Control key,
- click once on the Mathematica application (.app) and then
- select "Show Package Contents".
In the folder that opens,
- open the Contents folder, then
- the MacOS folder.
Double-click the MathKernel icon and a stand-alone kernel will launch.
Linux:
- In a terminal window, type and run
rehash
and then
math
What happens?