Greetings!
I am unable to find a scripture that says the scriptures are "complete." Perhaps you are referring to the spirit embodied in Revelations 22:18 or even Deuteronomy 4:2??
In any case you misunderstand both what JWs believe and the meaning of God's Word.
Both of the scriptures above have reference to the addition or deletion from the specific set of laws in the case of Deuteronomy and the prophetic vision and "scrolls" of the book of Revelation. It is a common misunderstanding among many to extend these prohibitions to the teaching of the entire Bible itself. Indeed no bible writer even under inspiration is likely to have set forth such a prohibition since none could have known definitively (unless they indeed were under Divine inspiration) that THEIR words would be the last to be inspired and thus properly included in the accepted canon.
(Here would fit nicely a larger discussion about the Bible canon itself and its arbitrariness but that is a digression I leave to others better qualified than myself.)
JWs do believe the Bible is "complete" but not in the sense that you are setting forth as your premise for your "question." The Bible is complete in the sense that through application and proper understanding of God's word, it holds all that mankind needs to both live today and tomorrow (All Scripture is inspired of God and benefiicial....etc.) and more importantly gain eternal salvation through exercising faith in Christ Jesus.
JWs do not pretend to ADD to God's Word, they merely pretend to understand it more clearly than anyone else. To that end, they claim a special relationship via their divine channel, the so-called Faithful and Discreet Slave Class. JWs do not receive new "revelations" of the TRUTH in the sense that perhaps prophets and apostles did in the Bible. Plainly if they did then they would not be wrong upon occasion or need to make "adjustments" to their understanding.
Like the apostles and, in fact, others thoughout the bible record who had to adjust their thinking upon receiving new teaching from God, JWs claim only to have an inside track today on receiving that continued teaching and so from time to time do make adjustments to their theology (belief system).
For those who choose to follow them or rather the leadership of the Organization faith is placed in God that He is leading them generally in the right direction but no one that I am familiar with believes the leadership to be infallible. Faith thus is not to be placed in men but rather in God.
-Eduardo