Getting Your Partner into Couples Therapy
First of all, you are to be commended for your desire to do whatever it takes to improve your marriage. Seeking help is the first and often the most difficult step.
In terms of how to include your wife in the couples therapy process, you have a few options. It may be that you haven't fully communicated your reasons why you think seeing a therapist might be a good idea. Try asking her calmly and gently, and tell her why you need her to participate. If talking has become difficult, try writing your wife a letter with your reasons why you want to save the marriage. Use the letter as an opportunity to reach out to her tell her some concrete, positive things about her that you appreciate now or have appreciated in the past (taking care of the kids, doing the bills, etc.). Think of this as an olive branch to her and as a life preserver you are throwing out to your floundering marriage. If you truly feel that your marriage is in danger of not surviving, a measure such as this one—in which you are open and giving of yourself—is critical.
If even after you have given her the letter, she refuses to accompany you to couples therapy, go see the therapist yourself. Tell your wife beforehand of your plan, and resist using your attendance at couples therapy as an opportunity to "show her up" or prove you are the more committed mate. Instead, just let her know that though your first choice is to have her go with you, you don't want to push her, and you will be going on your own, with the hope that she will be joining you soon. Your wife might need to see you take the first step on your own, in order for her to believe you are committed to the process of improving the marriage.
Let the therapist know what is going on. He or she might offer to call your wife also and extend an invitation to her. Even if your wife never chooses to come to therapy, there are many things you can learn and apply in order to improve the marriage: specifically, your part in it and how you can make changes for the better.