It seems you may be referencing those dlls from the GAC. In that case, since they won't be available in AppHarbor's build machine's GAC, you will have to include the dlls inside the code you upload.
One easy way to do it, is to create a libs folder (at the same level where your solution is would be a good location), and place all the required dlls there.
Then remove the existing references to those dlls from your current project, and add them again but from that libs folder. Also, for each of those references, it may be necessary for you to set the CopyLocal
property to True
(if you click on the reference you will see CopyLocal in the Properties Tab below the solution explorer).
More info on Copy Local can be found here: http://msdn.microsoft.com/en-us/library/vstudio/t1zz5y8c(v=vs.100).aspx
That way, AppHarbor will be able to build the code, since it will look for those dlls in that libs folder that you have just uploaded along with the rest of the code.