Hogwarts was just as concealed:
“But Hogwarts is hidden,” said Hermione, in surprise. “Everyone knows
that . . . well, everyone who’s read Hogwarts, A History, anyway.”
“Just you, then,” said Ron. “So go on — how d’you hide a place like
Hogwarts?”
“It’s bewitched,” said Hermione. “If a Muggle looks at it, all they
see is a moldering old ruin with a sign over the entrance saying
danger, do not enter, unsafe.”
“So Durmstrang’ll just look like a ruin to an outsider too?”
“Maybe,”
said Hermione, shrugging, “or it might have Muggle-repelling charms on
it, like the World Cup stadium. And to keep foreign wizards from
finding it, they’ll have made it Unplottable —”
—Harry Potter and the Goblet of Fire
Nonetheless, the delegations from the foreign schools had no trouble finding it. One presumes that Dumbledore could make the location available to the students from Beaxbatons and Durmstrang, if need be. He certainly seems to be able to remove the protective spells around Hogwarts:
As they flew over the dark, twisting lane down which they had walked
earlier, Harry heard, over the whistling of the night air in his ears,
Dumbledore muttering in some strange language again. He thought he
understood why as he felt his broom shudder when they flew over the
boundary wall into the grounds: Dumbledore was undoing the
enchantments he himself had set around the castle so they could enter
at speed.
—Harry Potter and the Half-Blood Prince
A similar situation probably holds with Durmstrang and Beauxbatons: they can simply alter or remove their spells to allow in foreign students. In particular, given that the delegations only arrive on a single day, they would only need to lift their protective enchantments for a very short period.