You're right that time is a dimension but it's a single dimension. Normally we can only observe the 3 dimensions that define space (x,y,z) and time is constant. But under certain conditions time is not constant and is the 4th dimension (x,y,z,t). You've pointed out a very good example about dimensions. If you take one step forward or 5 steps forward you've moved in a single direction (dimension) and changed your position but not the number of dimensions. When something travels close to the speed of light the passage of time is affected. It's called time dilation and time begins to slow down. The closer to speed of light you get the slower time passes.

This is how time is a dimension, depending on how fast you are moving affects how fast time goes by. I have to admit that I don't know much about time travel or parallel universe but there are interesting articles on the web which you can find by typing "time travel" or "parallel universe".

I hope this answers some of your questions.