I've agreed with her about the thieves guild i've been to the dungeon with the sword only to find a blood stain and a shield no sword i've waited 3 days not counting the few other days i used for traveling to the ruin and some other quest i did after talking to her she wont give me her quest im lvl 11 what am i doing wrong? i got her sword before i even met her on my assassin

To receive Mjoll's quest you must be at least level 14 and have some level of positive standing in Riften. When first approached, she will tell you about the sword, but with no quest being activated. When you are of a sufficient level and standing, you can ask about the sword, starting the quest allowing you to retrieve the sword.