Yes, you can apply to Canadian High Commission in Buffalo if you have alrady lived in USA for more than one year. Meaning you have to make full application as a skilled worker. Doesn't matter if your wife and kids are in Nepal.. all it matters is the principal applicant who lived in USA.

I am afraid, with your spouse's I.Sc. education you may not gather a single point under the Adaptability Factor. They look for more like trade certificates/dipoma. Perhaps overseer education from Engineering Institute of Pulchowk can give her 3-4 points... but I.Sc. means nothing learnt in terms of tradescraft. See this and try yourself taking the Self Assessment:

http://www.cic.gc.ca/english/immigrate/skilled/assess/index.asp

Regarding what kind of work experience letter you migth have to submit... read carefully the document checklist. This tells you exactly what kind of letter you will have to ask from your past employers.

http://www.cic.gc.ca/english/pdf/kits/guides/E37048.pdf