Make sure the project to be referenced has been built. If Build Automatically is not enabled on the Project menu, then do the following:
Select the workflow solution project in Package Explorer.
Click Project > Build Project.
The target\install folder should contain a solution file after a successful build.
Determine whether to include or reference the solution project:
An included solution does not update with changes made to the original component project. Do the following to include a solution:
Make sure the parent solution project appears in Package Explorer.
Click and drag the solution file from the target\install folder of the solution project to be included to the root folder of the parent solution project.
A referenced solution updates if the original solution project is changed and rebuilt. Do the following to reference a solution:
Right-click the parent solution project folder in the Package Explorer pane, and then click Properties.
Select Project References from the list.
Select the check box beside the solution project you want to reference in the workflow solution project.
Click OK.