Hi Dugley and welcome to the Netduino forums
There are actually a few ways to deal with this.
The detailed way: This way all source code is available and yet in another project
1. extract the zip -somewhere- to your system. Let's take D:\Netduino as example, so extract the framework to "D:\Netduino\netmftoolbox"
2. Create a new project for yourself
3. Click in the top menu: File -> Add -> Existing Project
4. Browse to "D:\Netduino\netmftoolbox\Framework\Toolbox.NETMF.csproj"
4a. Do the same for "Framework\NET\Toolbox.NETMF.NET.csproj" if you need networking classes
4b. Do the same for "Framework\Hardware\Toolbox.NETMF.Hardware.csproj" if you need hardware classes
5. Click in the top menu: Project -> Add Reference -> Projects
6. Select all projects and press OK
7. Add "using Toolbox.NETMF;" at the top of your project
7a. Add "using Toolbox.NETMF.NET;" as well if you need networking classes
7b. Add "using Toolbox.NETMF.Hardware;" as well if you need hardware classes
The DLL-way: This way no source code is available but no other files are added to your project
1. extract the zip -somewhere- to your system. Let's take D:\Netduino as example, so extract the framework to "D:\Netduino\netmftoolbox"
2. Create a new project for yourself
3. Click in the top menu: Project -> Add Reference -> Browse
4. Browse to "D:\Netduino\netmftoolbox\Release\Toolbox.NETMF.dll"
4a. Browse to "D:\Netduino\netmftoolbox\Release\Toolbox.NETMF.NET.dll" as well if you need networking classes
4b. Browse to "D:\Netduino\netmftoolbox\Release\Toolbox.NETMF.Hardware.dll" as well if you need hardware classes
5. Add "using Toolbox.NETMF;" at the top of your project
5a. Add "using Toolbox.NETMF.NET;" as well if you need networking classes
5b. Add "using Toolbox.NETMF.Hardware;" as well if you need hardware classes
Simple way: This way all required files will be embedded in your project
1. extract the zip -somewhere- to your system. Let's take D:\Netduino as example, so extract the framework to "D:\Netduino\netmftoolbox"
2. Create a new project for yourself
3. Click in the top menu: Project -> Add Existing Item
4. Browse to the required classes, for SMTP_Client: "netmftoolbox\Framework\Tools.cs", "netmftoolbox\Framework\NET\SMTP_Client.cs" and "netmftoolbox\Framework\NET\SimpleSocket.cs"
5. Add "using Toolbox.NETMF;" at the top of your project
5a. Add "using Toolbox.NETMF.NET;" as well if you need networking classes
5b. Add "using Toolbox.NETMF.Hardware;" as well if you need hardware classes
Hopefully this helps?