I created a title, edited into the timeline, and it won't show when I play the sequence; then I tried the "render at position" by right-clicking on the title clip and select from the menu, it worked. But at other times, it won't work, the "OK" button on that popup window grays out. Am I doing it right or I'm missing something here? I'm using MC3.0.

You need to ensure that "render on the Fly" is checked under the CLIP menu.

 I did have that option checked (turned on), although it's actually under the "special" menu. What I found is that if I right click the title clip and go "render at position", it works; then if I move the position of the title clip, I may need to re-render the title clip, but some other times I cannot do the re-rendering because the option is just not available: the OK button on that "render at position" dialogue window is grayed out.

 I still haven't got this title playback issue solved. Any ideas? It's annoying that every time I move the title clip, I need to render it again. The render on-the-fly command under Special menu has been selected, but it doesn't seem to be working.

Check in your Render settings (Or maybe General, can't remember), Do you have render settings set to highest quality? If so, that will cause this. Make sure they get set to "Quality Set in Each Effect".

 Thanks for the reply. I checked the render setting, it was set to "quality set in each effect," so that's not the culprit here. I also tried the other two render settings but the problem persists.

Well, those are the only two options in the software that would cause this. After this, you need to try a new user setting. If that doesn't work, try a new project. If no Joy there, then you need to reinstall the software.

 what can go wrong in the user setting though?

Just about anything. It may not be the user settings but this is always a first troubleshooting step.

 Maybe I'm missing something here, but if you have clips on layers below the rendered title clip, moving that clip changes it's positional relationship with the lower layers and will always require a re-render. Can you elaborate?

 I'm not sure what you mean "clips on layers below the rendered title clip," what I have is a video clip on V1, and on V2 I have a title clip created with the basic title tool.

 Yes, so if you have a video clip on V1, a title superimposed over it on V2 and you render the title clip, the render includes the picture information directly under it on V1. Change the position of the title clip on V2 and you have changed what picture information will now be under the title clip and will require a re-render. But, if you have that simple a timeline construct, you really don't need to render the title clip. It will play in real time.

I believe that's the crux of the problem here Larry, it's not playing for him in real time. 

When the title isn't rendered and won't play back, what color is the dot on it in the timeline?

 Cristolin:

If the render settings are set for "quality set by each effect", the title clip should have a green dot, indicating real time playback.