I dunno if this is really an "Alternative health" answer....but I think folks who say they are unable to visualize may be trying too hard, or missing the forest for trees, or something like that...
Anyway--calling an image to mind is something almost everyone can do. If I say, for example, "Think about a Stop Sign" you will call to mind the image of a stop sign. Then, if I say, for example, "now concentrate on the letter "S" on the sign" most people can do this, for an instant or so, at least..... Visualizing deities is the same--one needs to be familiar with the image, and call it to mind. What's hard, much of the time, is thinking of oneself as that image. If such is the case, than recall that it is one's mind that is the nature of the deity, --what is called to mind, as an image, is of a piece with the mind itself.
That advice has helped me, in the past--perhaps it will help someone else.
"Absolute Truth is not an object of analytical discourse or great discriminating wisdom,
It is realized through the blessing grace of the Guru and fortunate Karmic potential.
Like this, mistaken ideas of discriminating wisdom are clarified."
- (Kyabje Bokar Rinpoche, from his summary of "The Ocean of Definitive Meaning")