Selenium IDE is basically a add-on for firefox. You can install this using this link.
Here is a step by step guide to install Selenium IDE:
Step 1: Go to the link mentioned above and Click on Add to firefox.
Step 2: Click on Add.
Step 3: Click on Ok.
Step 4: Choose what do you want to proceed with.
Step 5: I have selected Create a new project. Give a name for your project. Click on Ok.
Step 6: You can start your testing now.
Hope you found it useful.