According to Plants for A Future, some alders have edible catkins, but they have a very bitter taste.
http://www.pfaf.org/database/search_name.php?ALLNAMES=Alnus
The ones with edible catkins apparently are:
White Alder (Alnus rhombifolia)
Red Alder (Alnus rubra)
Sitka Alder (Alnus sinuata)
Mountain Alder (Alnus tenuifolia)
American Green Alder (Alnus viridis crispa)
All other alders are marked "Edible Uses: none known" - which doesn't necessarily mean they aren't edible, just that PfaF can't find any sources that say they are. However, if they were poisonous, it would probably be known.