I have just bought a Doro 8075 additional handset for my Doro 8075 phone. I can't get the new ....

hello frnds

Sometimes handsets need to be placed in the base unit to register.

If it came with it's own cradle and charger then it should work. Perhaps it's a faulty one.
Make sure it's fully charged before you try programming [about 16hrs] then try again.
If it still won't log on to the base unit take it back to the shop, take your user manual, make sure you don't leave it there.
You can then with the aid of the book go through the progamming sequence with the sales rep so he/she can see your doing it correctly. If you are doing everything you should, and have done at home ask them to change the unit, it may be faulty.