For BrokerOpen you need a different function type:

typedef int (*BROKER_OPEN)(char* Name,FARPROC fpError,FARPROC fpProgress);

and then

Code:
BROKER_OPEN BrokerOpen = (BROKER_OPEN)GetProcAddress(h,"BrokerOpen");
if(BrokerOpen) {
 char brokername[50] = "";
 long version = (*BrokerOpen)(brokername,NULL,NULL);
}


You can see on the "Broker Plugin" page which functions you have and how they must be typedef'd.

For your purpose you only need BrokerLogin, BrokerAsset, and BrokerHistory.