Why are there no dictionary entries for ですので? I've searched JED, Tagaini Jisho, and Rikaichan. It seems to have an equivalent meaning with だから、but I'm not exactly sure. However I've heard it being used by my Japanese president, my Japanese teacher, and others.
Looks like you're interested in ですので at the beginning of a sentence?
Then yes, that ですので is a polite version of conjunctive だから, meaning "Therefore", "So", etc. The casual equivalent is なので.
However there is a controversy as to whether ですので/なので at the beginning of a sentence is correct. It seems most Japanese dictionaries do not explain this kind of ですので/なので. Announcers at TV Asahi state that なので is incorrect at the beginning of a sentence but ですので is polite and acceptable.
I personally feel なので/ですので should be avoided in written documents. But they are common and natural in spoken language.