how to create a multiplier or adder for floating point

The term floating point in the thread title is misleading as you clearly referred to fixed point in your initial post. To answer the original question, there's no difference between integer and fixed point in add/sub, but the result scaling is different in multiply. I suggest, that you try by pencil and paper method first. Generally, when multiplying two 10 bit signed numbers, you get a 20 bit result. It has to be scaled to a 10 bit number to keep the original format. At the MSB side, you may want to apply saturation to handle possible overflows, at LSB side, some bits are simply cut off. Altera has no MegaFunctions for this operation, you must provide it in your code. If you run short of FPGA resources, it can be meaningful to reduce the precision to 9 bit, which is the size of an elementary hardware multiplier.