I have created a tiefling fiend warlock. The issue is when i add spells to his character sheet they are only shown at the highest possible level, so i can only cast spells at the highest level using the highest spell slots. This is a very annoying issue and i'd like to know if anyone else is experiencing this.  

Warlocks always cast spells at their highest spell level, per the Class Features table.

That is how warlock spellcasting works: they only have  a certain spell slot level, which you can check on the warlock table.