I'm trying to understand what I'm doing wrong. I want to be able to enter the nether from the between lands so I edited the config.

I'll copy the part of the config I changed.

# Custom saplings or blocks can be specified here to work with the Swamp Talisman. Syntax is "modid:blockname:meta/dim", meta can be * for wildcard, if no meta is provided 0 is used. The dimension ("dim") specifies to which dimension the portal will lead to
S:portal_dimension_targets <
    minecraft:obsidian:-1
 >

# Betweenlands portals will only work in these dimensions
I:portal_dimension_whitelist <
    0
    -1
    1
    20
 >

Did I miss anything, because when I click, and I also tried to shift click, on an obsidian block with the talisman. Nothing happened.

what am I doing wrong

Your entry should look like this: minecraft:obsidian:*/-1.
There should be an error in the log that tells you if and why an entry is incorrect.