All of the alien races that visit Earth "use" (a few) other dimensions.
They are not from other dimensions.
The alien races can already contact us from other dimensions.
We don't have to wait until we can do it.
It is done by technologies and natural abilities, and the alien take care of it for us during our contact.
Spirits of people whose bodies have died can move through all of the dimensions at will by natural ability.
The alien races can't move through all of them either by technology or natural ability. 
I know only a little about dimensions from my alien friends, spirit friends and angels, and only in a simple way.
My understanding is that there are a few plus a few dimensions.
I do not know how many there are beyond that.
Dimensions are places within the same space as other spaces.
All dimensions share all in the same space.
I would not say that anybody exactly exists in a dimension and not another.
In my mind, we all dwell in the same ground dimension, and may in one way or another visit another dimension.
The alien people who visit Earth, because they have greater experienced capabilities beyond ours yet both naturally and technologically, use some other dimensions to travel vast distances in because different dimensions have different values of time.
The resulting affect is similar to running up a rising escalator.
We may visit another dimension or just look into another dimension from this one.
We can do these things three ways - by our natural abilities, by technology, or both.
By technology, all Earth's visiting races have access to "a few" other dimensions.
Also by natural ability, some of the most advanced visiting races also have access to a few more dimensions than that.
None of our visiting races have access to all the dimensions they are aware of there being.
I asked them, and they declined to answer my question of how many dimensions there are.
They did not tell me if they knew how many there are, either.
Spirits of people who have died can move effortlessly through all the dimensions.
I don't recall anybody telling me about the angels using other dimensions.