Well, its not just any black hole they believe we can enter. scientists believe there are 'kerr holes' these are rotating blak holes, might not have a singularity (the bit where you everything gets completely crushed) so if they dont have a singularity, they think that it is safe for people to travel through without being killed, they beleive on the other side they will exit through white holes which do the opposite to black holes and isntead of taking all matter it, it throws it out, so they believe this could be a means of time travelling, or just moving through space to 'parallel dimensions'
about moving at the speed of light, i dont really think it can be possible because i think the amout of G's that would be there would be crazy and you would simply be crushed and flattened into whatever you were sitting in.
also you dont necessarily have to be going at the speed of light either you can go close to the speed of light and still reach time travel because as you got closer to the speed of light time would start to slow down, but not as quickly as it would if you were at the speed of light.