Can you apply in person, In Canada for Visitor extension?

I am here in Canada visiting friends on a long vacation. I arrived initially in October 2010. I've gone back to the states twice, i am Citizen of USA

I was given one extension in February, until June 25, 2011. I left and came back in during May and the Border agency did not want to admit me, based on
her saying I was too close to reaching the 6 month rule, which I cannot find. The immigration office in my town said there is no set rule.

I would like to stay longer and realize it's too late to apply online and receive it in time.

Can I go to a walk in immigration office in Canada to obtain an extension, or if I do submit application online, does that allow me to stay longer legally while I wait for reply? If I do have to leave Canada, can I go to an office in the US to get a new visitor visa as a walk in?

Thanks for any info.