Brussels. It is just like a lot of lobbying groups in the US have their office in Washington D.C.

For example United States Chamber of Commerce, Pharmaceutical Research & Manufacturers of America, Business Roundtable and NCTA The Internet Television Association all have their HQ in Washington D.C. (all 4 are in top 10 largest lobbying groups in the US)

That's where Brussels is located. It makes sense to have your lobbying office near the offices of the institutions you want to lobby. Less time spent travelling.

Brussels