When u get a hit in the TLB,it generates the physical address or the frame address that maps onto the appropriate frame in the main memory...so TLB access time + main memory time..
But when its a miss, that means u have searched the entire TLB for the page number but u didnt get so TLB access is there.. Now u have to access the page table to search the appropriate page number so the page table access time is included. Now comes the mapping to the appropriate frame.. Once u do that main memory time wud be included.. Hence the equation.. Remember that If u don't find the page in the page table it will generate a page fault then u have to fetch it from the disk...