、。
Please jot down my address so you don't forget.
Literal
Forget [if-と] must-not [because-ので], my address [object-を] please-memo.
忘れるといけない is a conditional + いけない construction: 'if [you] forget, it won't do.' ので links this warning to the request. メモする is a suru-verb built on the English loanword 'memo.' The polite ~てください makes the request. A naturally structured, practical sentence.