Do you have an option "edit word" on the popup menu above the word? You can open a new tab with all the info, then edit the mem from there (or add a new one).
Maybe it's only visible to curators, in that case, please forget what I wrote.
i think it used to be possible.. also if one looks at ones own mem list, one can't edit them from that page.. again this used to be possible.. i have reported this to ben in the past, and he answered that it's something that will be possible at some point.. i guesss they are just swamped with other enhancements and improvements in the pipeline..
You should be able to edit your mems during a learning session. If you put your mouse over where your username is, a small "Edit" button will pop up.
It doesn't show up immediately after you make it though. So if you create a mem, hit submit, and realize you accidentally made a typo, the edit button won't be there yet. Any time after that first time though it should be there.