I would say EAT, EAT, EAT.  Any good workout program should have some sort of cardio in it.  How much you need to run depends on what your goals are.  The heavier you are, the harder it will be to run without having potential joint issues.  I would say talk to a certified personal trainer, have him assist you in coming up with a program that meets all of your goals.

I am assuming that you would like to gain more muscle as opposed to fat.  In order to gain any type of weight, you must eat more.  With that said, you still want to incorporate cardio into your program as 1) its good for the body and 2) you want to burn excess fat from your diet so that the weight that you gain is lean body mass.  In order to accomplish this, you must do high intense running or interval training with appropriate recovery periods.  You can easily see that track runners are very lean and muscular whereas long distance runners lack a significant amount of fat OR muscle.  Keep in mind that interval running will take some adaptation and your body will need more time to recover.   Additionally, it is often hard to motivate yourself to sprint so I recommend that you find a partner to challenge you or a heart rate monitor to keep you honest!

If you want Muscle weight...the formula is simple..."RUN and LIFT LOTS" and "EAT EVEN MORE." Minimize on high fats. Maximize on protein and veggies.

Good luck!

Figure out your basal metabolic rate (how many calories you use just staying alive) and add the calories you expend doing every day activities and daily exercise (running, weight lifting, climbing stairs at work, etc.)

Then make sure you are eating more calories than you are burning if you want to gain weight. I would recommend no more than a 350-500 calorie per day surplus to assure you are gaining quality weight (lean tissue).

Too much running will inhibit muscle gain. Running intensely or distance causes the production of the hormone Cortisol. Cortisol inhibits muscle growth.

If you are wanting to gain weight because you are unhealthfully (is that a word?) thin, I suggest not running for some time and focusing on strength training along with a good weight gain diet.

Once you have achieved a healthy weight it will be easier to maintain and running can come back into play. 

If you are already a healthy weight, but want to pack on some more muscle, run easy 20-30 minutes 3x per week. Do not combine running and resistance training sessions.