Apple Logic Pro /LogicExpress Discussion

Wonder if anybody can help. I have just bought a G5 Dual 2.3 and Logic Pro 
7.1 I can not find any way of "installing" third party plug ins. I have checked the 
ones I have ( Waves Platinum for AU for OSX ) and they are valid for Logic 7. I 
have put several plugs in Applications , but they are not even appearing in 
Logic Audio Units Manager. Im probably missing something really simpe 
here. Im also having the same type of problem trying out "demo" versions of 
some new AU instruments e.g. X treme FX and MOTU MX4 Soft Synth ( that is 
appearing in Audio Unit Manager but will not open, so maybe its not valid for 
Logic yet?) Pleeeeeeease Help. Thanks. Paul