Why is it so difficult to find a simple calculator for my El Capitan iMac? The Dashboard isn't working

The easiest method is for me is to launch the app, then locate its icon in the dock. Click on the icon and hold the button down. A menu should appear. Click on 'options' and select 'Keep in dock'.

Here's an excerpt from an Apple article that provides additional information on how to add an app or file to your dock.

macOS Sierra: Use the Dock

Add or remove Dock items

• Add an item: Drag it to the Dock. Drag files and folders to the right of the Dock’s separator, and apps to the left. If you moved the Dock to the side of your desktop, drag files and folders below the separator, and apps above.When you drag an item to the Dock, an alias to the item is placed in the Dock. The original item isn’t moved.
• Remove an item: Drag the item out of the Dock until you see Remove.Only the alias is removed from the Dock; the original item isn’t removed from your Mac.

I expect you'll find different work flows for different people. I typically go to the "Today / Notification" pop out window (click on the icon at the top right of your screen and this window will slide out from the side of the screen) for simple one-off calculations. If I have a need to keep the calculator on the desktop, I do something similar to what iCare & rkaufmann87 have suggested: I hit the command key & space bar & type in calculator, it is typically the highlighted option returned, hit enter and the application is launched. It will stay on your desktop until closed or until the Mac is rebooted. If you want it permanently on your dock, do as iCare has mentioned.

can't you use the normal built in one ? desktop calculator?

If you use Spotlight you can almost instantly find any app (or file) installed on your computer. Simply hold down Command + Spacebar and type the first few letters of the app and it comes up.