If you want your IUD removed, you need to have doctor do it. It is positioned and if you try to remove yourself, you could cause serious damage to your cervix and uterus. The reason you can't find the string is because they are not meant to be removed outside the doctor's office.
If you can't find the strings you need to go see your doctor. It may have repositioned up higher and your doctor may need to remove it anyway. You should always be able to feel just a little bit of the strings, and should be checking for them on a monthly basis (as I was directed by my OB/GYN).