You would want to navigate to %appdata%/.minecraft/bin and run minecraft.jar with the Java Runtime Environment.
Install the Java Runtime Environment (JRE) for your operating system, then open the Structure Planner Jar file.
you probably haven't downloaded Java Runtime Environment. no its lag, computer fing up, or just java (the answer above) have you resarted it tho?
Runtime Environment
No. Minecraft is written in Java, which means you either have to have a JDK (Java Development Kit) or JRE (Java Runtime Environment) to run the game. You can download and install the latest Java from their website (see related links).
The java runtime environment can be downloaded from several websites. Among them are Oracle, Java, CNET, Softsonic, Eclipse, AfterDawn, Soft32, and OldApps.
The J2SE Runtime Environment is a platform that enables quick development and execution of Java applications. It was developed by Sun Microsystems.
Because you mine and craft and it copied jetpack builder and dwarf fortress, on a totally unrelated note, what is your pc runtime
A big yes - Minecraft is a Java programmed game, using Java to run the actual application. Without Java Runtime Environment, Minecraft would not be functional to your computer, it would be no more than a downloaded file.
.net is framework is a runtime environment for running .net projects..
Go to sun.com you can download any version of JRE there
If it lags to much, you need to download the newest version of Java Runtime Environment, then set it back to far. It should stop lagging
why i can' install JRE on nokia n73