@ verbosewater - Take a look at the tutorials, the should give you a reasonable foundation to build on.
What you need to do is start a new gadgeteer project, you should then get a designer which shows you a mainboard, if the mainboard is not the mainboard you are using the you can drag the correct mainboard from the toolbox which by default can be found on the left of the designer. Once the mainboard is there, yo can find the motion sensors module n the toolbox and drag that to the design surface, you can then link the module to the mainboard by either right clicking and selecting the option to auto connect the components or you can explicitly drag a link from the module to the board. You will see the supported mainboard sockets turn green when explicitly linking the module.
Doing the above will ensure that your project includes all the relavnt libraries to be able to use the module.
Just to help you get a basic understanding of the problem, here is a simplified explanation.
The error says that you are either missing an assembly reference or a using directive. In .NET and .NETMF assemblies are containers for code, typically .exe and .dll files. So to use the code in an assembly you first need to reference the assembly in your project, once you have the assembly reference you can access the data types exposed by that assembly.
To avoid naming conflicts, data types in assemblies are contained in namespaces, so when you want to use a data type you need to know what namespace and assembly it is in. A namespace is really just a dotted prefix to the name of the type, you can avoid repeating the namespace everything by including a using directive near the top of your source file which basically tells the compiler to search the namespace for any types it cannot match in the code. The can be multiple using directive and the compiler will search them all.
Assemblies can contain multiple data types in multiple namespaces and a namespace can span multiple assemblies.