かたい帯を上手に結ぶことができない若い女性が多いため、柔らかい帯を去年より３０％増やしたと言っています。
From http://www3.nhk.or.jp/news/easy/k10010984931000/k10010984931000.html
I would translate it as follows (without ため):
They say that there are many young women who can't tie a hard Obi, from last year the number of soft obi has increased by 30%.
I can't find an entry on jisho on ため and I can't remember having seen it on my textbook either :/
It means "because". It could be replaced with から or ので. It's used in writing more than in ordinary speech. Sales of soft obi have gone up "because" many young women can't tie stiff obi. (I'd go for "stiff" rather than "hard".) ため can also mean "for the sake of", as in 人民の、人民による、人民のための政治 "Government of the people, by the people, for the people"
So looking at Jisho.org, I think that you are looking for definition 3 (consequence; result; effect).
My translation is very close to yours:
It is said that the usage of soft obi(s) has increased by 30% since last year, a result that comes from the fact that many young women cannot tie a stiff obi.
Yuuichi Tam's comment is very pertinent, you can also translate that as 'Because,' but ため has a more formal context than から/ので。 The reason you may have difficulty finding that translation stems from the fact that English has phrases that have similar meanings to because. Check the link for examples. The truth is that ため is more formal and would be better translated to by reason of ___ of only just to keep the formal feel. If you don't care about formality quite as much, just plain because is adequate, and 100% accurate.
Retranslating that sentence using because we get:
Because many young women cannot tie a stiff obi, it is said that usage of the soft obi has increased by 30% since the last year.
Hope it helps.
