[daeron80]

At the top of that dialog (the one that comes up when you hold down N during program launch) is a Current Engine dropdown list. That's where you choose your Tascam interface. If it doesn't appear in that list, the drivers are not properly installed.

After choosing the US-800, choose a H/W Buffer Size. In general, use the smallest one for tracking and the largest one for mixing, if needed. Small buffers give you low throughput latency but require more CPU, which makes for a better tracking experience. Large buffers take the pressure off the CPU at the expense of I/O latency, giving you more power for mixing large sessions.