Complex sentence: Dassault Systèmes thus got started in 1981 with only one customer (Avions Marcel Dassault) and 25 engineers who didn't know how to sell and market their product.
Split into simpler sentences: Dassault Systèmes thus got started in 1981 with only one customer (Avions Marcel Dassault). They were only 25 engineers and didn't know how to sell and market their product.