What cpu cooler do you have installed?
All stock coolers come with pre-applied paste.
Most after market coolers come with a syringe of paste you have to apply yourself, a few have pre-applied paste, eg: Corsairs AIO watercoolers.
Are your temps decent? If so there is paste applied. If you leave off the paste you will get pretty awful temperatures, pretty much any paste will work magic compared to no thermal paste, even using mayo as your paste will cut 27C off compared to no paste based on the HW secrets testing! http://www.hardwaresecrets.com/article/Thermal-Compound-Roundup-February-2012/1490/5