Where is the SD slot located on my iMac?

If you have a Micro SD Card, then you need to put it into a Micro SD to SD adapter before you can put it into the iMac's SD card slot.

https://www.officedepot.com/a/products/6728541/SanDisk-Ultra-PLUS-microSD-Card-32GB/?

If you have some other type of Camera Card, then you will need to get an external USB to Multi Card adapter.

for some examples > https://www.officedepot.com/catalog/search.do;jsessionid=0000-JHrg8RZad_HW-5Yn0Q8LXq:17h4h7cou?

What year model iMac do you have?

see > https://support.apple.com/en-us/HT201634

The card reader is on the back of the models that have a card reader.

The latest models without built-in card readers, require using an external USB card reader.

What iMac model do you own???????????????????????

If it is a new 2021 24" model they do not have one, otherwisw please look on the back.

You're welcome.