Basic design structure

The first non-obvious basic is that the standard design file using Capture as the front end, is the project, not the design.  A project file (.opj) contains information on what type of project you are using (such as simulation, PCB or just a schematic) as well as what other files are being used for this project.  The design file (.dsn) contains all the data for the schematics, as well as a cache of all the symbols used in the design.

THE FIRST COMMON ERROR IS STARTING A NEW DESIGN, INSTEAD OF A PROJECT!

When you do this, you get assigned a plain vanilla project file for your design.  You won’t have any simulation tools or PCB layout tools available for your project.  Similarly, if you just double click the .dsn file in Explorer, Capture starts up without any simulation or other resources, and creates a blank project file for use with the design.

ANOTHER COMMON ERROR IS NOT STARTING A SIMULATION PROJECT!

If you just create a new project but don’t specify an Analog or Mixed Mode Simulation project, Capture doesn’t give you access to PSpice.

To fix these problems, you need to create a NEW project that has simulation tool configured that includes the old .DSN file
. 
Go through the New Project Wizard (File>New>Project) and select the Analog/Mixed Mode Simulation choice. 

Specify the directory containing your existing .DSN file. 

In the next screen, it will want to know if you want to configure that existing .DSN file into your new project. 

Say yes, and you are back on track.