Tenpenny tower

The Grudge, Tenpenny tower is at the South West corner of the map. And you could be able to find Tenpenny Tower if you head to Paridise Falls, and the Slavers give you the quest to enslave Susan Landcaster or some name like that... I have NO idea about a hollow rock though.
You'll reach Tenpenny Tower if you start from Springvale School (Just north of Megaton) and you hike westward to Jury Street Metro Station, from then you must head north Paridise Falls ( this may take you a while) and then you'll get a quest from the slavers out side the entery showing you where Tenpenny Twoer is. Hope theis helps =)

Tenpenny Tower is Southwest of Megaton. If you see a big Tower that looks like a Hotel, with windows, it's that. You can't miss it.

Ok where ever you heard that must be wrong because it doesn't exsist because I haven't found it with walk throughs and with a freinds help. But you can find it east or south east from any part of the map.

I know where the hallow rock is. It isn't necessary to find it, but I like to, as it contains a sniper rifle, which is fun to have that early in the game, some ammo, stimpacks, and a few other things. Anyways, from the front of Megaton (outside) circle all the way around, hugging the wall of the city. Look at the rocks as you pass them, but be careful, as there are mole rats that hang around back here. Now, keep heading around until you see a rock surrounded by three skinny dead trees. Be attentive, as it's very easy to miss.

Tenpenny Tower is easy to find. Start the quest, "The Wasteland Survival Guide" (If you blew up Megaton, Moira will be in underworld as a ghoul) and go until Moira gives you the option to check out some old tech down at the Rob-co factory. Go there, and Tenpenny tower will be right next door. Don't forget to use the intercom to get in, you may need to bribe the guard with some caps.