First of all you should download installation package for: "IAR Embedded Workbench for ARM" at http://www.iar.com/Service-Center/Downloads/
(size-limited evaluation license)
When the installation is complete you need to register online (you can use any name and country, but you need valid e-mail). You will get a registration key by e-mail.
Now you can activate the product with the key and run the main window: Click the menu item "Create New Project", select the language and MCU architecture (ARM, C/C ++ language) in the dialog window: You will get a new project that contains empty main.c file.
Next step is to configure the project properties for your MCU (board) STM32F3Discovery (stm32f303xx). To do this, click the "Device" button and choose the correct device for you project from the list (click button with "+" sign). It is recommended to include the library named: "Cortex® Microcontroller Software Interface Standard (CMSIS)". Most likely it is already installed in the IDE and is available in default "Include-paths". For project configuration, you will need to configure include-paths to MCU and SPL header files. Whether your project uses the Standard Peripheral Library (SPL) then you should add this "#define": USE_STDPERIPH_DRIVER
On the Debugger option page, make appropriate settings for the debugger you are using (ST-LINK): On the Debugger->Download page, select the "Flash Loader" option: When your board is connected to the PC via USB-cable select SWD option. When you use a device programmer then select JTAG option:The CMSIS and SPL libraries provide some abstracted functions for MCU setup, input/output, system startup code, and peripheral setup functions.
DONE!Do not forget to add tool-chain files to your project. These files are required to build a project in the IAR tool-chain and are part of CMSIS and SPL.
startup_stm32f303xc.s - assembler file from tool-chain, that creates the C runtime environment (stack and heap), initialises the processor, defines and setups interrupt vector table, and jumps to main().
system_stm32f30x.c - here you will find the CMSIS SystemInit() routine and other customised code for your board that configures crystal frequency (HSI/HSE) at 72 MHz, PLL setup, SRAM configuration etc.