Once you've written your computation results to a buffer (or texture) you can Map() the buffer/texture and read back the results.
Note that for some resource types (depending on the creation flags) you may first need to copy your results to a staging resource and then Map() this intermediate resource.
This link (even though for DX10) contains some pointers to related topics: http://msdn.microsoft.com/en-us/library/windows/desktop/bb205132(v=vs.85).aspx
One thing to keep in mind: If you read back the results in the same frame as the computation happens you will probably introduce stalls in your CPU-GPU communication which can lead to performance problems. So wait at least a frame before reading back data. Even better would be to issue a query after your Dispatch() call and then asynchronously ask DirectX if the query is finished so you know your computation you did before is finished as well.