そうびは 「ぶき」と 「ぼうぐ」に分けることができる。 ところで おやくそくだが 「メニュー」でそうびしないと意味はないぞ。
I understand the second sentence as roughly "By the way, if you don't equip them in the menu, they don't do anything."
I'm assuming this is お約束. At first I thought it might be literally "I promise you that X" to add certainty, but that didn't seem to account for the だが.
Another definition I've found for お約束 is "something expected or predictable". Does this essentially make it mean "It shouldn't be surprising, but X"?