First, you must convert the mesh to a format that Poser can read. Wavefront OBJ is preferable, although 3DS, LWO, and DXF are also options, depending upon your Poser version.

If you don't already have it installed, download the Tempest game pack plugin. This will allow you to export meshes in Quake 3 format. From there, you can convert from Quake 3's MD3 format to something more Poser-friendly using an application or utility like MilkShape, Ultimate Unwrap3D, or LithUnwrap.