I'm afraid without more info I can't give you an exact answer.
This is a pretty open-ended question. Whether 74% is good depends on your target score, on how the percentages break down across difficulties (eg 74% accuracy on very difficult questions is much better than on very easy questions), and on how long it took you to solve the questions.
If you use the GMATFix App (check out my signature at the bottom for more info), open up the Report Card to get narrow feedback of your performance; the App compares your performance to that of everyone else on the network and gives you a grade for accuracy and another for pacing based on the target score you've set.
Click "Report Card" when the App loads
You get an overview of your performance
Pick a more targeted report card to see specific topics
You can also take a diagnostic quiz on my site (go to the "Resources" page at GMATFix.com) and send me your results (you will see that option at the end of the quiz); this way I'll have more information to give you better feedback. I will go over the quiz and send you a detailed report.
As far as how to improve your accuracy, it again depends on what exactly is holding you back; not all problems have the same solution. On the Resources page of my site, I have a Self-Study PDF document that discusses how to make the most of the Manhattan GMAT materials.
Sincerely,
-Patrick
