how do you access the borg missions i"m a RA upper half and i still have not received any.

Regular Borg missions are give to you at RA3 and upwards, and only after you've gotten the mission from admiral Quinn to go to Gamma O. For this to happen, you need to spend enough skill points. Once there, more will be assigned to you... all 4 of them.

For the STF's however: Below your minimap, theres a button called "PvE Queue", click and open that one, and find the missions Infected, The Cure and Khitomer Accords...