Stuck at "Hemlock's map with altar"

[Mathematica]

Hi

I have some problems with the altar. I've read the walkthrough, and the altar is next to the clamshell right?
When I hold the mouse over it I don't get the description "Altar", only "????". And when I try to use the Hemlock's map with it, Guybrush just says "No".

I'm loosing my mind here! Can anybody help me?

[Fester Shinetop]

Nope, the Alter has no clamshell next to it. Keep looking

[Mathematica]

Oh, silly me... I was convinced it was it, but I'll try looking around.

Thanks!

[Ecto Cooler]

Try veering to the path on the right, near the jungle entrance.