There are three ways to import from AC3D into Second Life:
1) Sculpted prim maker
Using AC3D's popular and easy to use interface, you can create objects for Second Life quickly and without many of the restrictions that other 3D modelers enforce.
2) Box builder
Simply create your model in AC3D then watch your creation assemble inside Second Life.
3) Upload ANY polygon model in Second Life
Our Second Life Triangle Builder lets you upload virtually ANY 3D model inside Second Life.
The movie below details the Second Life Triangle Builder:
Sure using Triangles is object intensive but it works and it works well (our previous maps in Second Life were created using our own triangle builder).
If you have the space and it fits into the maximum object limit then this really does open up object import into Second Life.
You can helpfully try AC3D free for 14 days so grab a trail, install the plugins and start importing those models into Second Life...